I have two 3-year old cats (sisters). Until recently, they both used the litter box without a problem. A few months ago, one of them started pooping on the floor outside the room where the litter box is. I suspected it was because she couldn't fit through the cat flap very well to get to the litter box (she has gained some weight and the cat flap was really small). I've tried everything since then, leaving the door open, installing a larger cat door, getting a larger box, using the cover for the box, taking the cover off, changing the litter 2 times a week instead of just once, trying a new type of litter, scolding her when I catch her, etc. But nothing helps. She does use the box to pee and I know that this is not a medical problem. I use wood pellets as litter which they have used since they were born. It is possible that she's just suddenly become picky about how clean the litter is and I'd have to change it at least every second day to keep it spotless.
Although, I think I can keep the litter clean enough, I suspect she is now just in the habit of going on the carpet. She mostly does it when I'm not home or when I'm sleeping so it's hard to catch her. Can anyone tell me how I can break her of this habit?