The "German" problems possibly were more complex: Mussolini threatened to switch sides again like the Italians did in WW1 already, The Nazis had massive supply problems because of the British sea-blockades and when Hitler invaded Russia, Stalin stopped sending trains full of steel, obviously. Rubber also was a big problem and fuel even more, i believe. Stalin just put his industry behind the Ural Line and could wait for General Winter and also used the bad infrastructure (streets and so on) Russia had at this time.

I don't believe the Nazis could have occupied Russia with the Allies supplies Russia got from Convoys through Murmansk.

But the Britons and US did let them bleed a lot of souls intentionally in delaying a second front in Europe with only invading Sicily very late and the Normandy even almost too late, so to say.