I also have this problem and it's usually Ziggy. She can leave some real stink bombs and I do feed them all high quality grain free cat food. I've heard that leaving the poop uncovered is a sign of dominance. I think it's also a case of just being lazy. Many of my boys don't bother to cover up their pee either. My RB Storm used to go into the litter box and then he'd cover up everything that the other cats left uncovered. He was a very tidy cat.