What I expected to happen was...:
Villagers find their beds at night. Villagers will breed when there are enough beds.

What actually happened was...:
Villagers seem to freeze up in certain areas at night. Even though they have more than enough beds they do not return to them. This only happens with one or two villagers here and there. If you can nudge them enough towards their registered bed I've noticed that they will eventually begin walking again and find their way back to it. 

It seems almost like they walk out of their usual path and can't get back onto it again. If they do not return to sleep the game thinks that their is not enough beds and the villagers will not breed.