Here's an idea that doesn't break canon too badly

[raketenjagdpanzer]

A post I made a while ago called "the care and feeding of nuclear weapons" touches on the maintenance issue. One component of modern nuclear weapons, at least American types, is polystyrene. Same stuff your $.99 picnic cooler or chick-fil-et cup is made out of. As it turns out it's a great way to quickly generate plasma to amplify a nuclear explosion, so they fill the hollow space between the actual core and the nosecone with it.

Well, the alpha radiation emitted from the core tends to melt the polystyrene, reducing it to a black crud that can gum things up, and so it needs to be periodically swapped out when it shows decay. If that's the case with US nukes there could be part of the problem right there.

[Olefin]

the US nukes are in the custody of MilGov - they didnt leave them in the silos or on delivery vehicles per several module sources - so it may be that the warheads are still in good shape but they lack the ability to deliver them

whereas with the Soviets they may be sitting in unattended silos or abandoned mobile units and thus neither the delivery vehicle or warhead works