I have a siamese that had a problem with that before and its not a pleasent one. apparently siamese have an even more sensitive sence of smell than other cats which ive heard is 10 times that of ours. I first of all went from changing the box every 2 or 3 days to EVERY day and cleaning it with anti-bacterial soap and lisole and then the fun part of getting theyre scent out of the carpet or whatever it is they are peeing on, get your carpets cleaned and buy some of that cat urine spray stuff from your vet it will help.