Im glad you brought this subject up. My DS is 6 1/2 months and has half rolled once. He also hates being on his back so i tend not to put him there. DD was much more advanced with milestones.
My clinic nurse is worried about everything that isnt text book and wants to assess him in 6 weeks and if still no results then she wants to send me off to Physio. I call her the referral freak. Lol. Glad im not a first time mother other wise id think there was something wrong.

All babies develope at there own time like the other girls have said. Try not to stress and just enjoy your little one while she is still young. They grow so fast so dont try to speed the process up iykwim.