BW...I wouldn't worry at all hun. Jack didn't roll from tummy to back till 4.5 months old and from back to tummy till 6 months, and he is just fine! You saw how he was getting around at the last meet up. His is now walking constantly and trying to RUN, lol, and he's only 11 months.
Babies will all do things at different ages and it's nearly always "normal".