Some thoughts:
- Russia's nuclear success was based on espionage - Klaus Fuchs (German, then UK, then at Los Alamos) basically handed the USSR a complete bomb design document package.
- Bomb design as such - any nuclear physisicist could build a weapon, maybe not the most efficient but quite nasty anyway.
- The most difficult barrier to building any nuclear bomb is the economic cost and technicalities of refining weapons grade plutonium and/or uranium sufficient for a critical mass.

Germany did not succeed because Hitler caused many leading European physicists to emigrate to the UK or USA. He also issued a policy to the effect that nuclear/quantum physics were "Jewish Physics" and therefore not to be promoted or taught at German universities ! Nor did Germany - unlike the USA - have the economic/industrial capacity to refine weapons grade uranium or plutonium. It may be argued that Hitler in effect presented the atomic bomb to the USA to the detriment of the  German war effort.