Try moving the bed around. If you put the bed in a completely open area, will the sim use it? If yes, then there's something around the bed, maybe something in the way, that stops your sim from using it. I know I just had this problem with stairs and it turned out to be pictures on the wall that were in the way. If the sim still won't use the bed, I'd say there was something wrong with that bed. I'd sell it and get a new one.