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Babies don't always reach milestones at the same time. Maybe she just doesn't want to yet. As long as she can lift her head on tummy time, she should be developing fine. Maybe put a toy she wants a bit out of her reach to encourage her to stretch.
My daughter is 5 months old and has only rolled over a hand full of times. she does everything else normally. Instead of rolling over, she grabs her feet and rocks back and forth and that is how she (kind of) gets around. She also HATES tummy time. She is really getting to the point where she wants to be sitting up all the time, so I prop her up on her Boppy or put her in her Bumbo seat. She should have a dr's visit coming up, so ask the doc. He/she may be able to give you some peace of mind.
I think my son rolled over a grand total of four times before he started crawling. And then walking. And then running. Now he won't sit still. As long as your baby's healthy, happy, and responsive, I wouldn't worry about it too much.