how heavy is your baby?
my doctor says it's quite common nowadays that babies only roll-over even after 8-9mths if they are heavy, especially if they have fat legs
some babies can flip over even they are 4 or 5mths, so it all depends on the parents whether they are brave to let the baby to flip as well as the baby wants to flip at that time or not, cos some babies do not want to flip at all, as they don't feel comfortable.
so don't worry, take your time! let your baby flips naturally! :)