I want to second everyone else who has posted and say that every child develops differently. My first son didn't roll over until both our Parents as Teachers rep and our pediatrician told us we needed to work on rolling. (That was at 8 or 9 months, I think.) It turns out that we just couldn't stand to let him struggle for anything and so he didn't have any motivation to roll. (We handed him everything he wanted.) I think that happens a lot with first children- parents are so involved in everything, that they tend to do too much for the baby/child. Don't worry about it. Practice rolling her yourself a few times a day (make it a game), give her lots of tummy time, and start placing toys just a little out of her reach. She'll roll when she's ready.
Remember- the guides to development are just averages (that means most children hit that milestone earlier or later than the milestone says.) There is so much growth, especially of the brain, before children reach the age of three that each child is wildly different from the next. You pediatrician knows the warning signs, if she isn't worried, you don't need to be.