C.F.
You must make sure it is clean and doesn't smell or they will not go in the litter box. My cat would do that and when he did I knew the litter box was not clean. There is some cat box freshener by arm and hammer that is real good. It is like carpet fresh for the litter box. Also make sure the cat can get to the litter box at all times. If something is in the way and they can't get in then they will go where ever they can.