When my cat had kittens recently, and the kittens started litter box training, the mommy stopped using it! I was horrified, she started using our bathroom sink instead.
All of the people here said they thought she probably didn't want to share. I went and bought an extra box, and confined her to a room with it for two days. She has been much better and the problem is solved. I think Donna is right when she says to get another box and see if that does it. It worked for me, and I really think cats dont like to share!!