It's often for me. Some AIs just get broken... I don't know why. In my last game for egzample Mexican, Colombian, Brasilian armies worked greatly ( they used tactics against my troops, etc.) but French, Prussian armies were standing still and Spain had 1 stack that stand still and rest of their army was moving just fine.

Also it seems like it's a bit save-reload fixable, but on the other hand reloading game sometimes releases nations that weren't there x__x