There's two main reasons for the reduced effectiveness of German offensives throughout WW2, those being the increasing scarcity of the resources necessary for Germany to carry out a major offensive, and the Allied Powers development of counters to Germany's mobile warfare tactics. The combined arms mobile warfare that Germany employed thorought the war was initially very formidible, but it wasn't invincible, and the Allies were eventually able to combat, and eventually counter the Blitzkrieg. Also, misconceptions about the Blitzkrieg still persist to this day that bolster its perception as some hyper mechanized super efficient method of warfare, so lets stamp out some of those along the way too.

First lets tackle resources. A major offensive requires a superiority in almost everything relative to your enemy, often times on a scale of 2 to 1 in order for an offensive to be successful. In terms of German offensives in Poland and France, Germany was simply able to muster more men and material, get those men and material to where they needed to be faster than their enemy, and when the Germans were outnumbered or outgunned, relied on their impressive momentum and speed to encircle and isolate otherwise united and strong military formations to oppose them using robust radio communication between units to coordinate more effectively than their enemies. The resources Germany had available to them were always limited, with Germany not having much in the way of cobalt, copper, and most importantly, oil in order to keep a modern war machine going, and mobile warfare requires a LOT of resources to keep the perverbial engine going.

When it came to Poland and France, Germany was sufficently supplied by the Soviet Union to carry out its offensives, with the Molotov-Ribbentrop Pact containing a comprehensive trade agreement alongside the political agreement to carve up Poland. The Soviet Union supplied Germany with millions of tons of grain, cotton, phosphates, and that sweet precious oil to keep Germany supplied and its soldiers and civilians fed throughout the early war, but also made Germany reliant on Soviet resources to continue prosecuting its war against the Allies. Hitler became increasingly paranoid of the amount of influence Stalin had over Germany's regime through the Soviet oil supply, knowing that even the expanded Reich couldn't produce enough oil to keep its army sufficiently fueled. So Hitler made the decision to simply invade the Soviet Union and take the resources for himself to make Germany self-sufficient, and launched Operation Barbarossa.

The planning for Operation Barbarossa was hamstrung by one obvious fact, the immense distance Germany would have to cover as it drove deeper into the Soviet Union. When war against the Soviets starts, all the oil shipments keeping the Wehrmact fueled will cease and it becomes a race to get to the Caucasus oil fields before Germany runs out of fuel for its army. Even with all the men, tanks, artillery, and supplies Germany could muster for the invasion, Germany's supply and logistics simply couldn't allow for the German army to advance all the way to Moscow and the Caucasus fully supplied and uninterrupted, with the German supply corps making it clear that they could only keep the German Army supplied as far as Smolensk, after that the advance would need to halt for a few weeks in order for supplies to catch up before advancing again.

In the end, Operation Barbarossa failed due in large part, but not entirely to the failures to keep the German army supplied. As the Wehrmacht advanced deeper into Russia, shortages of everything including clothing, ammunition, food rations, spare parts, and fuel along with the resistance of the battered but still standing Red Army stopped the Blitzkrieg, and threw it back outside the gates of Moscow. After the Battle of Moscow, Germany simply did not have the resources available to move 3 entire army groups simultaneiously across such a wide front, with each German offensive getting progressively weaker and weaker due to Germany's supply limitations. Case Blue, the invasion of Southern Russia and into the Caucasus, only had the resources to move Army Group South and was wholesale defeated at the Battle of Stalingrad. By 1943, Germany didn't have the resoruces to even move a single Army Group anymore, with the Battle of Kursk only having the supplies to move pieces of Army Group Center and South, only to be defeated again, not by supply shortages, but by a true Soviet counter to the Blitzkrieg, where the Wehrmacht was completely unable to break the Soviet defensive line.

After Krusk, Germany had no realistic ability to stage a major offensive again. the Wehrmacht would attempt one more counterattack on the Western Front in the Ardennes Forest in 1944, but extreme supply shortages and the strength of the Western Allies halted the German advance at the Battle of the Bulge within 2 weeks.

Now lets talk about counters to the Blitzkrieg.

The first real cracks in Germany's mobile warfare tactics were observed in their invasion of France; where during the German breakthrough into French territory caused the German Panzer divisions to outpace their footsoldiers to the point where there were large gaps of open territory between the Panzers and the soldiers and supply corps, leaving the Panzers vulnerable to being surrounded and cut off. During the Battle of France, a halt order was issued for the German panzers to stop advancing in order for the soldiers and supply corps to catch up. The allies picked up on this and showed that if the German panzers advanced too fast, they risk sticking their neck out too far and being surrounded.

It was also learned that because the Blitzkrieg relies on armored spearheads piercing weakpoints in enemy defensive lines in order to advance, a general "hugging" of the shoulders on each side of the breaking point in the lines also leads to the reduced effectiveness of the breakthough, and leaves the Panzers that broke through open to encirclement if the break caused by the spearhead cannot be fully exploited.

Finally, finding out where the advance will take place using military intelligence can allow for the defenders to effectively prepare for and anticipate the enemy advance. The Soviets were aware that the Germans would attack Kursk for months in 1943, and filled the salient with pillboxes, minefields, barbed wire, and anti-tank traps and oordinance in order to make the Germans wade through a sea of their own dead in order to advance.

Also, while we're here, lets smash a couple misconceptions of the infallibility of the German war machine when it comes to Blitzkrieg. People and books talk at length about the Blitzkrieg as some fully mechanized form of warfare that only involves tanks and trucks. The German Wehrmact was actually severely lacking in terms of supply infrastructure, with much fewer motorized trucks to carry supplies compared to the Allies, and relied heavily on horse-drawn carts in order to maintain a supply line when railroads failed. The German's mobile warfare tactics are also played up to be this well-oiled machine with every unit working in perfect harmony with each other. While radio communication did make coordination more effective, it wasn't perfect, with German panzer units and supply trains often colliding with each other due to mixed up communication, causing massive traffic jams that slowed the advance, most famous of which being during Case Blue where German units were involved in massive traffic jams while advancing towards the caucasus.

Sources:

When Titans Clashed: How the Red Army Stopped Hitler by David Glantz

Blitzkrieg: Myth, Reality and Hitler's Lightning War - France, 1940 by Lloyd Clark

Blitzkrieg: From the Ground Up by Niklas Zetterling

Edits: apparently I spell blitzkrieg differently every other time, fixed those typos.

After the fall of France, even senior German staff were saying to themselves "Hmm. That shouldn't have worked", but because it did, they thought they'd try again in Russia. Yeah, that was a bad move.
See for example this interview with the Panzermuseum Director, Ralf Raths, should be timestamped to 13:20. "They did what they did very well, and they had the luck they needed not to fall flat on their faces.... We should never have won, that shouldn't have worked". Elsewhere he says "They went to France expecting WW1 reloaded and got Blitzkrieg. They went into Russia expecting Blitzkrieg and got WW1 reloaded"

In 1940, the Germans had the good fortune of going up against an incompetent French senior leadership (I realise this is a general statement and some French leaders were as good as any other, but as an overview, French leadership were just not up to the task). It wasn't that the Germans were far superior, as much that the French threw away what chance they had. Reading Karl-Heinz Frieser's The Blitzkrieg Legend is an eye-opener as to just how close the Germans came to failure and just how much the French screwed up. I'm not talking doctrines here, but execution of doctrine. Much as people lambast French doctrine for being 'inferior' to German (an argument which completely ignores the larger picture of national situation), had the French leadership applied it correctly, they could well have ejected the Germans from the Sedan Meuse crossing.

In later battles, the Germans tended to come across enemies who were at least willing to try to make a decision and fight, unlike General Billotte who the British Chief of Staff had to grab by the collars and shake him out of his stunned inactivity.

The next problem is the "Blitzkrieg" wasn't exactly new. German doctrine was more correctly called Bewegungskrieg, the concepts of which dated back to the mid 19th Century. Everyone was fairly familiar with the German appreciation of wars of maneuver, even if they didn't necessarily ascribe to them for their own doctrines.

Thirdly, logistics, never a German high point. Germany's was a horse-drawn military. Barring the much-touted panzer units, most German assets trudged along the same way as they had in the 1870 war and WW1, this remained so through to the end of the war. The German logisticians figured, when calculating the invasion of the USSR, that they could support up to about an 800km dash before running out of steam, an estimate which proved fairly prescient. Unfortunately, the operational commanders didn't hold the logisticians in as high an esteem as they probably should have and tried anyway, with the expected (from the logistician's point of view) result.

So it's not as if they didn't try to use it throughout the war to finally win WW2, they tried to use it and eventually lost a fight they shouldn't have expanded in the first place.