Don't worry - I think as long as you are giving her tunny time she has the opportunity to do it if she wants, you probably find she can but can't be bothered - I have a friend whose baby is 28 weeks and she doesn't roll but can sit up and pull her self to her feet when holding mums fingers, it's not because she can't roll it's because she is quite happy on her back. Remember all babies are different, my son is 23 weeks and can roll back to front but can't get back again but I'm sure when he's ready he will.
My daughter Lara is 18wks and is still trying very hard to roll over but hasn't quite got to the point where she will go all the way. When i saw my HV on friday she made a point in stating that by now she would have expected her to have mastered it, this has made me feel a little anxious that maybe i'm not encouraging her enough?? She has both tummy time and time on her back a couple of times a day and has this week started to sit up for a short period of time on her own. Is it just me or has my HV got a point any advice would be grateful.