Something I found on the Babycenter website, under the "when to be concerned" section:
If your baby hasn't figured out how to flip one way or the other by the time he's about six months old and isn't showing any interest in getting around by any other means, bring it up the next time you talk to your doctor or health visitor. Babies develop skills differently, some more quickly than others - and some babies never really take to rolling over - but if your child hasn't moved on to sitting and trying to scoot or crawl instead, ask your doctor about it. Keep in mind that premature babies may reach this and other milestones later than their peers.
So, I guess have a look at the whole picture, rather than just pinpointing on the one milestone? What does your baby do when you put him/her on their tummy?
my daughter is turning 1 in about 10days....she hasn't crawled, scooted, started flipping at 9 months, but sits REALLY well from the age of 7months...she's more passive I think, so she can literally sit through our dinner without a problem at all! my sister's 7month old is pushing the walker I bought my daughter (whom doesn't even show interest in the toy) and he's walking up a flight of stairs that is quite long (13-15steps) all unaided! I think the reason is because he has 2 older siblings whereas my daughter is an only child...less stimulation and motivation to move perhaps?!? I don't know...chinese nurses at QMH say she's overweight (my own doctor doesn't think so)...so I'm like, she'll do it in her own time...what can you do!