De-Nazification is basically a myth. A very small amount of Nazis were ever punished for their crimes. The rest merely embraced the new order and integrated themselves into the system. In order for "De-Nazification" to totally fail, as in the German people rise up to re-establish the Nazi system, you would have to have the Allies treat former Nazis with a much rougher hand. This way your average German/Nazi feels inclined to risk their life by rebelling against the occupation, rather than merely being glad that they were allowed to walk away scot-free like OTL. But good luck getting the Allies to take this occupational mindset. The narrative was always that Nazism was evil, but that the German people were good (as opposed to the Pacific theater, where the Japanese themselves were seen as evil). Even German PoWs were treated with an incredible amount of dignity by the Western Allies, yet alone average German citizens. A Morgenthau Plan is simply ASB and would have zero support from the public in America, Britain, or even France. And the Soviets, for their part, believed that fascist terror was the problem, and once a socialist system was established, the German workers would be free of their sins.
And, of course, any rebellion is going to have to have outside aid as well in order to succeed, and good luck with that.