I would up the tummy time a lot, chat to her and sing face to face, and S-L-O-W-L-Y roll her onto her back, letting her go a little earlier each time if you know what I mean?
Someone once told me that you never see adults not being able to roll, crawl, or walk...as frustrating as it is, it will happen eventually. My little one will also not roll from tummy to back, but ive seen her do it in her sleep...i think they just get frustrated
When she did that i would lay her on the floor let her go to her belly, when i realized she tried to roll back over i helped her a little and within a few days to a week she was rolling back over
I was told if they can roll one way then physically they should be able to roll the other way. She's probably just getting frustrated. When my daughter was doing this I did a TON of tummy time during the day to really try and encourage her to figure it out. It took a while, probably about two weeks, but eventually she got there.