To Use Litterbox Cat doesn't like type of litter. Many litters have strong deodorants, which many cats do not like. Litterbox is not clean enough. Bad location for the litterbox. More than one cat using the same litterbox.
Solutions Try changing brands of cat litter.
Change the litterbox every other day, and scoop daily. Make sure you thoroughly wash the litterbox, with a good cleaner. You should have one litterbox per cat. Placing aluminum foil on the spots that the cat has begun to urinate on. Take the cat to the vet for a physical exam to check for a bladder or kidney infection.