If you're trying to stay hollow, then you're probably not driving your heels. I hate when kids are too hollow on casts because they'll have a hard time getting higher than horizontal-- there needs to be a tight arch on the way up before you hollow. As far as clear hips, I do clear hip hands from a cast horizontal, and the key for me is to push back immediately after the cast and drop toward my knees. Once you have a strong drop, the rest of it feels like a back extension roll (so being able to do those with a good body position with straight arms helps, too).