There are many reasons a cat may not like his litter box. One of the most important reasons, and thankfully the easiest to remedy, is that the litter box is just plain dirty. Our feline friends will put up with a lot in terms of lax litter box cleaning habits, but at some point every cat will stop and say “No way!”
Some cats, just like some people, are more bothered by unsanitary conditions and will stop using their litter boxes sooner than other cats might. Unscooped, dirty litter boxes are the cat equivalent of that public bathroom with toilet paper all over the floor and unflushed toilets — you wouldn’t want to use that either! An extension of this problem is having too few litter boxes for your cats — the rule of thumb is at least one more litter box than the number of cats in the house.
Litter boxes should be scooped at least once or twice a day, and it’s even better if you can get to it as soon as your cat has finished his business. There are self-cleaning litter boxes available that use a sensor to tell when a cat has entered and then left the litter box. While these are great at keeping up on scooping duties for people, they tend to scare at least some cats away from using their litter box and would not be a good choice for a timid cat. In addition to daily scooping, it is important to regularly change the litter box (twice weekly for non-clumping litter, monthly for clumping litter) and to wash it with soap and water. Plastic litter boxes should also be replaced once a year, because the scratches they tend to get during regular use can hold odor and debris.