i had the same problem...
you should buy the cat repelant spray, you can buy it in most pet
stores... it stops them peeing inside.
once they pee somewhere, no matter how much you wash it they will
still smell their scent and keep peeing there.
the spray makes them not pee in certain areas.
you should also try using different litter in the cats litter tray.

hope this helps.

Have you taken your cat to the vet?

If not, then this is the first step towards resolving this issue... you
need to rule out a physical issue.

Call your vet and see about having the urine tested for crystals or
infection. Until we learn to speak cat or the cats learn to speak 'people'
the only way they have of indicating a problem is to engage in behaviors
that tell us something is wrong.

You want to eliminate the possibility that this is a medical problem. You
can waste a lot of time treating this as a behavioral issue and get nowhere
if the problem is a medical one whereas as medical problem can be diagnosed
relatively quickly in most cases. If everything checks out normal then you
can worry about tracking down what stress is causing this behavior.

As others have said it's important to remove the smell from where the cat
has already urinated. Use an enzyme cleaner as the usual household cleaners
won't remove the odor well enough that the cat can no longer smell it.

W