Hey S. G! Don't stress too much, it's probably nothing to worry about, all kids do things in their own time. I'm a Physical Therapist and my own son took forever to crawl and walk...I think at around 7 months is when he also started rolling all the way over and started to be better at sitting alone...plus he did the combat crawl almost til he was one year old before he really got up onto his hands and knees. As a Physical Therapist I'm telling you this was a total nightmare hahah because I was doing everything to get his tummy time in and keep his gross motor skills on track...but he just had such a laid back personality, plus he was always a rather large baby (I think larger babies just don't move as easy :)
The thing to look at is her overall development...while my son wasn't on track with his gross motor skills he was advanced in his fine motor skills and speech, he was saying "mama" and "dada" by the time he was 6 months and shortly after that he was using words like "more" and "ah-done <--all done" for meals...he was speaking in full sentences at a year...when I tested him he always ended up age appropriate due to the areas he was excelling in so my pediatrician never worried...it took him awhile to walk (once he started crawling I was just so happy I could have cared less haha) but once he did he was walking down the hall...
he is still a very cautious child and timid about new activities, I can't even tell you how long it took before he would go down a slide by himself :) but I also never have to worry about him getting hurt or have to watch him like a hawk when we're out and about like some parents do. And it was kind of nice that he didn't move before he understood the word "no", it made it so we didn't have to childproof everything :)
Keep trying tummy-time, sometimes it works better when you have them on your tummy or you are down there with them so they don't just feel left in an awkward position...once my son figured out how to roll over tummy-time was a pain haha. Just keep stimulating and challenging her and I bet she'll do everything in her own time :) If you're really worried bring it up at your Pediatrician's office...but keep in mind, some doctors tend to freak out a little early if you ask me, I've seen many normal children in my office for that very reason :)