Rear 9" Drum Problem

Not fitting by a lot, or by a little?
As the drums age, they develop a ridge (the same Ridge that makes it a pain to get them off)
It may be hanging up on that

If it were me, I would pop the passenger side off and see if I can't find something out of whack
If the shoes are on backwards, I think that places the springs off center and forced the shoes open
(I know you said they are all on right, but I've known myself long enough to know if there is a problem with my car, it may be the loose nut behind the wheel)