-the cat doesn't like the type of litter you are using - if you changed it recently, this could be it
-the cat doesn't like the location of the litter box - if you moved it and did it too quickly, the cat might not use it
-there is another cat in the household that is not letting the 23 lb cat use the box - cats can be territorial about their litter boxes - if this is it, get a second box (or third, etc...)
-there has been a change in the household (a new family member, a new animal, etc...) that is upsetting or scaring the cat
-the cat has a urinary tract infection (UTI) - when a cat has a UTI, it hurts for them to pee, but they aren't smart enough to know it is a medical problem they are having - they actually think the litter box is causing their pain, so they look for someplace %26quot;softer%26quot; to pee - if you suspect this, take your cat to the vet
There are probably other options I am not thinking of right now, but again, before you can solve the problem, you have to figure out why it is happening.