My first was sitting himself up at 5.5 months and rolling both ways but my second just about figured out rolling both ways now at 6.5 months and nowhere near getting into sitting position by himself. It will all come in its own time.
I was helping mine roll for a while, when finally she learned to roll one way and then the other. But I have only seen her roll a handful of times, she just isn't bothered. It could be yours just doesn't want to roll. I'm sure it'll be okay!
Yep mine is 5.5 months old and she rolled over once at 4 months and since then no matter how many times we do tummy time, she hasn't done it. I've been told not to worry about it and to carry on doing tummy time to help strengthen her muscles.