Lizbud, you might want to try adding some cat attract litter to the other litterboxes and see if this will get them to start using them again. You can buy is at Petsmart and they do carry it in the store. Here's a link about it: Cat Attract Litter.

I've been having peeing issues and I had to add 3 more new litterboxes. I bought the cat attract litter and put some on top of the old litter and now they've been using all of the litterboxes. This stuff really works. Good luck.