Some have different potty habits. My youngest, Abby, has never covered anything she does. Well, ok, she did once and I was so excited about it that I came and posted about it!
But she is one that hates to get her paws dirty so she tip toes in, perches with 3 legs on a corner of the box, keeps one leg in the box and stands on her tippy toes, does what she needs to and leaps out.
Occasionally Chynna will go in behind her and cover it up, but Abby can't be bothered.
There is nothing wrong if they dont' cover it. That's just their habit. Besides, they know Meowmy or Daddy will come along and scoop it out anyway, so why put in the effort of covering it up when we're only going to uncover it again anyway?!