The Litter Box is dirty
Cats do not like using dirty litter boxes. Make sure you
change the litter often and wash the litter box with hot soapy water
at least once a day. Scoop out soiled litter regularly.
Your cat may not like the brand of
litter you have chosen
The location of the litter box is not to your cats liking
Don't put the litter tray in a busy location or near a noisy washing machine for example. Put the litter tray in a quiet easy to access location and don't forget they like privacy too!
Some cats do not like sharing their litter box with other cats while some don't mind at all. Maybe your cat is one of the former and requires his own personal litter box.
Ideally, one litter box per cat is recommended. If this is not possible then it is advised you provide the maximum number of litter boxes that you can cope with.
Multi-cat households pose numerous problems. Cats are territorial and by instinct they need to have their own space. In a multi-cat household this is often impossible and stress levels in the cats are raised. One cat will pee somewhere to show that this is their territory then along comes the next cat and pees over the top to show that this is their territory and so it perpetuates.
Pooping can also be a problem with cats in a multi-cat household defecating outside the litter box. It's sad to say but often there isn't a solution to urine and defecation problems in a situation where there are multiple cats living in a confined area.
If at all possible building an outdoor
cat enclosure may be of benefit.