My twins will be 7 months old in 10 days- they've been rolling for months but aren't sitting unassisted yet. Natalie will do it for short amounts of time (like 30 seconds) but B is nowhere near it.
E would not roll over back to belly, until after he was sitting at 7+months. He was late at doing everything gross motor-wise, in the first year. He was 3 weeks early, so I contributed it to that. Even being a PT, I was not freaked out by it, just figured he would get there eventually. A is 4 1/2 months and only rolling belly to back. Again, I am not getting hung up on it. They both had a lot of floor play time and that did not make a difference, at least for them. If there are concerns, the parents can talk to the pediatrician.
He didn't roll until the day before he turned 7 months. It didn't really bother me too much, because he was able to sit independently (if we placed him that way) around 3 1/2 months. He was slow with a lot of things to that point because of his prematurity, so I had learned to not expect too much too soon.
Re: Anyone's LO not roll over or sit up at 7 months?