My DS loved to roll from back to tummy for a while but couldn't roll back either. I've tried everything, but eventually she will get used to sleeping on her tum. She will learn to roll back, even though Blake knew how to roll from tum to back to start off with, he forgot after he learnt the other way. Took maybe 3 weeks and now he is rolling everywhere and half crawling. He was born 4 weeks early so I know what it's like!