Cats don’t like their tails touching anything when they go. So if you have an enclosed litterbox where the only way cat doesn’t feel like their ass is confined and touching stuff, they’ll stick it out the door. So you can try taking the lid off, or try taping a washcloth over the opening blocking 1/2 of the top so that the cat butt will touch that in hopes that this is more disagreeable than just being all the way in the box.