S.P.
My son is 6 1/2 months and does the same thing. He rolls over from tummy to back sometimes, but other times he just panics and cries. He had plenty of tummy time and can do both, but sometimes he just doesn't which is in bed most of the time. I asked my pediatrician and they are not worried and said he will figure it out and to just try to let him cry a little longer before I go in his room. He will either finally sleep on his tummy or remember that he can roll back. BTW, he rolled from back to tummy way earlier than tummy to back, so don't let the previous post send you into a panic. Every baby is different - the milestone markers are not the same for everyone - they are general guidelines.