My roommate and I had a problem with our cats not using the litterbox.
My rooomate bought a plastic litter catcher mat and put it in front so that the litter wouldn't get everywhere. The cats were constantly peeing on this mat! We got all different kinds of mats, thinking "they won't use this one!" but they just kept right on urinating on them.
Finally, we took all the mats away and just put the litter box right on the floor. Suddenly, the urinating only occured IN the litter box.
Apparently, the cats (or one of them) thought that the mats were PART of the litter box... LOL
So, if you use a mat by your box and your cat is going on it... take it away and see what happens
