Some cats stop using the pan because it is not cleaned often enough.  This sometimes happens with multiple cat homes.  In this case, more than one litter pan might straighten out the problem.  Some cats need to be within line-of-sight of their litter pan to remember to use it.  Other cats just seem to develop a bad habit of not wanting to use the litter pan.  This is a big problem, but there is a big solution.  Pet Quarters carries a cat litter designed by a veterinarian named Dr. Elsey and is called Dr. Elsey's Cat Attract.  This litter is made using an herb that will actually attract the cat to use the litter pan.  We get rave reviews about this product.  It is not an inexpensive item, but it lasts a good length of time, and every bag has a coupon inside for the next purchase.