honey I'm sure everything is fine, you are right they do things in their own sweet time and not to our promptings.
I had a similar feeling about DD my boss approached me and asked if I thought their could be a problem with DD's hips??? (I work in long daycare and my DD attends the centre where I work she's in the infant room) at the time she was the exactly the same age as your DD and was doing the same things could roll to tummy but not back again. She really didn't like pulling herself in to a sitting position and some how developed a way of sitting up that involved her doing the splits almost which I think is where the concerns came but two weeks later she started crawling and so far she seems to spend a fair time observing others before attempting I had the question asked of me again just after her first birthday as she hadn't started walking either and I swear as soon as my boss has approached me with her concerns DD started walking with in the next two weeks.
so I am really sure that you're DD will do things when she is ready