I've had fully retained sheds before with my colubrids and a mostly stuck shed when I first adopted my BP. Sometimes it seems to be a result of stress (such as occurring right after extensive vet procedures) or poor husbandry, but sometimes it just happens and you can't figure out why. I'm sure there are quite a few other possibilities that can cause it.
When I soak mine with retained sheds, I'll usually put something like the center of a paper towel roll in there; gives them somewhere to rest their head if they want, and also something to rub against if they decide they want to get the shed unstuck on their own.