Every baby is different in their development. Try making sure that your baby has plenty of tummy time so that he/she can develop muscles to roll over. You can use a bath towel and have another one rolled up. The rolled up bath towel should be placed to help the baby be prop up.
It honestly depends on their age. All babies learn and develop at different speeds. For instance, my 4 month old is not rolling over yet. If you're very concerned about it, I would bring it up at your baby's next check up.
