My 8 month old daughter isn't crawling?

My 8 month old daughter doesn't crawl yet. She is doing fine in every other aspect, she is healthy and can sit up fine. She also rolls all over, but doesn't crawl. She loves to walk in a walker and jump in a bouncer. She will also walk with you when you help her, but doesn't seem interested in crawling. I put her on the floor and she plays and tries to move around, but doesn't crawl for things. My son was crawling at 6 months. Why isn't she crawling? Should I be concerned? What can I do to help her? She gets lots of tummy time, and I really work with her, but still she doesn't crawl. Please help.