It sounds like you're bugged, although this is a new one that I've neither experienced nor seen on the official forums, so I have no advice as to cause or solutions. I didn't notice whether you are using mods or not. It could be a conflict if you are.
If this is a long running save, it could just be the inherent wonkiness that always tends to set in after a while. Unfortunately the engine can only handle a file up to a certain side, and once a game exceeds that, chaos can ensue. You can save the existing Sims that you want into your gallery, and import them into a fresh game. Unfortunately you lose some progress, but at least you get to continue their stories. Clearing your cache might also help.
Sorry that you're having issues, and that I don't have anything helpful to add. I hope that you find a solution.