The cats will not use the litter for a variety of reasons, and not always a simple answer to this problem. Here are some reasons to help you solve the problem.
Medical Problem
Medical Condition can cause your cat to pee inappropriately. Bring your cat to be checked by vet if you have this problem.
Diabetes
Diabetes can make your cat to pee more often. Sometimes, Your cat may not make it in the right place.
Brand/Type Of Litter
Your cat may not use litter box if you recently switched to different brand or switched to different type of litter. Once, I switched from sand type litter to wheat type litter, None of my 8 cats enter the litter box. You have to gradually increase new litter to mix with old litter.
Location Of Litter Box
Busy Location, Noisy Location are not a good idea to place litter box. Even you, may not use your toilet happily when it noisy outside.
Litter Box is dirty
Cat don't like dirty litter box. Scooped the litter box regularly to keep it clean.
Multiple Cats
Some cats won't share their litter box with other cats. Provide a litter box for each of your cats to reduce the toilet confliction. Think of yourself, There is no toilet available for your when you can't stand it anymore. It's armagedon!!
