My ds mastered rolling from back to front very quickly, at around 4 months. He is now 6 months and still can't go from front to back. Is he being slow to catch on or this this "normal"? And is there anything I can do to help him learn how to do it? He gets very frustrated when he's stuck on his front, and while I have been leaving him for a while like that so he has the opportunity to master rolling back, he doesn't seem to be getting it!