Ku Ku does the same...maybe related to the litter I've been using.
She used to cover it but since I switched it to feline pine, she stopped covering.
Still somehow "tries" to do it but probably the size of pellet, she "can not stir" it completely to cover.
Instead, she demands me to "clean it" as soon as she poops...
Interesting... About two weeks ago I started adding arm and hammar litter to their trays. He occassionally wouldn't cover it before that but very regularly now.... Hmmmnn