I have to agree with kwhite. I had a foster kitty who had the foulest smelling poo. Even when she went poo in the third floor litter box we could smell it on the bottom floor. There was nothing wrong with her physically. But I gave her some supplement and that helped a lot. I don't remember the name, but I got it at PetSmart. You may want to google "Smelly cat poo" or something like that.
Her poo smelled so bad that I considered adding on a "poo room" where she would have to go through a kitty door to the outside and go in a covered caged area outside! Luckily she got adopted :)