V.,
I wouldn't worry about developmental delays at this point. My daughter will be 11 months old on May 9th; she rolled over from her belly to her back once at 5 1/2 months, then once more at 7 months. She rolled from her back to her belly once at 8 months. I was so excited each time that I put it in her baby book, but they were total flukes! She just started rolling from her back to her belly in her crib last week (at 10 1/2 months). I haven't even seen her do it, but I think she might be using the bars of the crib to help her! She has hated being on her belly since day 1. I put her down for tummy time every day for a few minutes, so she was developing her strength (she could hold herself up at the angle she was supposed to at each age). I also would roll her half way to try to get her to roll, but she just didn't want to, so she didn't. I really didn't push it much, just a few minutes a day, and then when she got really frustrated, I would pick her up. She still doesn't crawl or roll on the floor; instead she scoots across the room on her bottom. She is very efficient at getting around in this way. I think babies just find what works for them.
Every baby develops so differently. There is so much pressure around to make sure your baby is doing things by a certain age, but I know plenty of people who have stories about a baby they know who didn't crawl until after a year or walk until nearly 18 months. I think continuing to put her on her tummy each day for a few minutes is important, but if she is constantly frustrated, it's probably only going to make her hate it more.
One of things we did, too, was lay on the floor or bed and put our daughter on her tummy on top of us. That way she was looking at one of our faces while she is on her belly. She could still push up, but it was certainly more fun than looking at the floor! You could also try putting a mirror in front of her when she is on her belly on the floor. She will love looking at herself and may even forget that she is on her belly (at least for a few minutes)!
Most of all, don't worry too much about what other babies are doing. It's hard, especially with milestones because we mothers love to talk about them. But every baby is going to be so different in the first 1-2 years, then they pretty much even out.
A.