The reasons for the failure of Barbarossa are legion and far to many for me to contemplate a full opinion here. Suffice to say that Hitler's notorious and banal "One swift kick!" quote shows that he thought the operation could be accomplished quickly, and to be fair it was a close run thing, thanks to Stalin's self denial and his paralyzing of his generals by fear and mistrust.
The Soviet Union was supposed to collapse after the first blows. The blows were there, and they were massive, but the Red Army and the Soviet system as a whole didn't collapse as was expected. It was there where German planning stopped. And they still had three and a half years to go.....That is the bones of my argument.