Lina was exactly the same way. I was a bit worried about it and then read in the Sears Baby Books that babies go from sitting, to lunging for things, to hands and knees, to crawling.
Sure enough, she's now, at 8 months, crawling a tiny bit. Last week she learned to go from her tummy to sitting up. At the same time she learned to stand for a couple seconds without holding onto anything.
So all that sitting is strengthening your dd's back and giving her practice stabilizing her core.
Lina also skipped rolling, okay, she went both ways like twice, but with great effort. She now flips from back to front and vice versa quick as anything and makes getting her dressed an exercise in trying to keep the tags lined up with her back.
Now, when I say crawl a tiny bit, I mean crawl forward like 2 inches and then lunge forward with an outstretched arm to try to reach things--and then squawk about being on her tummy. Or try to get her feet under her and stand up. It remains to be seen if she'll use crawling as a means of travel, so far it's just for getting to things to explore or things to pull up on.