That's why it comes with an extra ring. You're going too fast for your cat. I had the same problem, as I have two cats as well. Once was getting it just fine and the other wasn't comfortable going poop in the thing. I had to watch him and keep putting him on the ring when he had to poop and eventually he gave in because I wouldn't let him go on the floor. If you don't have the patience, toilet training isn't for you.
If your cat is pooping on the floor, the hole is too big. You have to use the extra ring to go back to a smaller size. You can't move too fast because it'll draw the process out even longer than it normally would be.