litter box problem

This problem has probably been answered 100 times already... but I'll ask anyway!

We have 2 cats. A 2 yr. old female (spayed) and a 6 mo. male (castrated recently.. an adventure in itself ) Our female had a problem with a UTI, and she's got a prescription diet and was treated with antibiotics. Her litter box habits are fine. Our male, however, has started leaving 1 "sample" outside the box every day. Everything else makes it in alright, but there's always 1 poop that's outside the box... why? and how do we fix it? Our boxes are off the ground (reason? 1 word: dog ) on tables (we have a large flat carboard box under each litter box, to prevent the litter from being sprinkled all over the floor!) and the poop is sometimes on the carboard box, or on the floor.

I would get a larger cardboard box so he can't kick the stool over the lip. Just cut a hole in the side of the box that the cat can get in and out of. If you still see it on the floor or table, then it could be the dog has gotten after it, (they think it is candy) because cats eat so quickly most of their food isn't predigested so that makes their waste pure protein. Or your cat is carrying his stool to the floor to mark his territory.

If he was actually dumping outside the box then I would be concerned of a health issue or a really dirty litterbox, but one piece usually means they kick it or carry it out. I am curious with two cats, how you would know who is leaving it out?

I would even add one more litterbox if you could.

Our cat has done that (one piece right in front of the box, everything else inside) every time I've used a litter he doesn't like, e.g. litter pearls, paper litter. That's his way of telling me he wants his ultra-clumping bentonite litter back. Maybe your cat is sending you a message.