The German tanks were put together by highly skilled craftsmen. Often, the parts were hand machined or finished for each tank. This meant that the tanks did not have interchangeable parts, and were difficult/impossible to fix in the field. Being unable to fix equipment in the field was a general problem for the German army, as a very large proportion of their trucks and other equipment was captured from other countries, and they had no spares.

Once the Soviets moved their factories beyond the Ural mountains, the Germans, who lacked heavy bombers, had no way to knock out the factories. They stood no chance against the Soviet T34s and tank destroyers at that point.

If you're interested in digging deeper into this kind of history, take a look at "The Second World Wars" by Victor Davis Hanson.