Well for starters many of the members of the allies couldn't do very much at the beginning of the war. For much of the war the only member of the allies who could actually attack Germany directly was the Soviet Union. America and Britain were limited to bombing raids or fighting in places like North Africa. The ability to mass produce tanks and equipment rapidly helped the Soviets massively, the western allies ability to mass produce helped them when they did get around to invading the continent.

Also the axis weren't really that successful for that long. America joined in the war in December of 1941 (so lets round it out and say 1942), by 1943 the Axis were in retreat on the eastern front after losing at Stalingrad (and they would soon suffer another crippling defeat at Kursk), they were losing badly in North Africa, and before the year was out Italy would be invaded and knocked out of the war. Not to mention the Japanese had suffered naval defeats at midway and the coral sea. So really a year after the United States had entered the tide had turned against the Axis.

Sources: Japan's Imperial Army: its rise and fall by Edward Drea The Second World War: John Keegan The Third Reich series: Richard Evans

Well first of all, most of Axis successes happened before America even entered the war. By the time the US and Germany declared war on each other, the Axis were already in control of most of Europe and stood at the gates of Moscow. WW2 started in Septermber 1939; the US didn't officially enter it until Decembter 1941. By the end of 1942, just about a year after the US had entered the war, the tide was already beginning to turn.

Secondly, it takes a long time for material and resource advantages to translate into real successes on the battlefield. At the very least, you need to convert existing production lines and reallocate resources used for civilian products into military ones. That involves changing your assembly lines, instructing your workforce to produce the new equipment etc. In some cases, the US didn't even have adequite equipment to produce. For example, the best tank the the US had at the time was the M3 Grant/Lee, which was quite frankly bad. So they needed to design, test and produce a new tank from scratch. In terms of manpower, it takes time to organize men into operational units with a heiarchy and a chain of command. It takes even longer to train them to fight - particularly certain specialist roles.

Last but not least there is the question of distance. All theaters of war happened to be far way from the US soil. The US needed to build hundreds of ships just to be able to send its forces across the ocean and to be able to supply them. It needed to build a large amount landing craft just to be able to execute a large scale amphibious operation such as the Normandy Landings. Of course the US also sent a lot of its equipment to its allies. But even in this case, it took a long time before this equipment went from the factory floors to the hands of British and Red Army soldiers.