Not entering war production prior to the Battle of France. The most common issue that haunted Germany at the onset of the war was a simple lack of stuff. Logistics was what held up Dunkirk and let the British escape. Logistics was what held up Barbarossa (contrary to popular belief, "General Winter" didn't halt the German advance. That was an excuse offered up- in all reality the winter wasn't that cold, and posed less of a problem for Germany than the Fall rains which turned everything into a muddy mess. German tank design severely underestimated the value of off-road capabilities, and simply didn't account for what ice and snow and freezing temperatures would do to tank treads.) The kinds of shifts in production Germany needed to do, it did far too late.

A close second- potentially a greater factor- was the simple fact that Hitler thought himself a mechanical engineer and a strategist. Just about every later war tank design that had Hitler's influence was an reliable piece of garbage. When speed and low profile were king Hitler demanded the Tiger- a large, slow vehicle- be designed. When the T-34 highlighted the need for a nimble, well sloped tank, Hitler turned the panther into something that weighed what the Tiger was originally going to weigh, didn't give it the suspension it needed, gave it a main gun that was awful (to be precise- the high velocity 75mm gun the Germans used quite commonly on tanks couldn't deliver a sufficiently good HE round- AT rounds could decrew a tank but a week later it'd have been welded over, a new crew put in it and it'd have a new coat of paint) and it's final drive guaranteed that with an average lifespan of 150km the tank couldn't engage in tank maneuvers. Comparatively, M4 Shermans that weren't knocked out in combat had a very real chance of driving from Normandy to Berlin without any major repairs.

Furthermore, Hitler intentionally left a level of disorganization where only he had the final say on anything, and that final say was often awful. His generals dreaded the idea of him in front of a map because he'd take the unit markers displayed on the table and simply move them, with absolutely no regard for what it takes to move an army even a single mile.