Grant, at 9 months old, still hasn't crawled yet. I think maybe he's too indulged - we bring everything to him. He has no need to get around, thus real motivation. Yet, we work with him on this skill as much as possible. It's paying off, as he's now scooting backward when he's on his tummy, and he can make real progress toward things by pivoting 360 and pushing backward with his hands.
See, I can get to my Sophie!
Mama, can you help me get up on all fours?
Oooh, look, I'm doing it!
Let me just get my Sophie over here... oops! I didn't know I needed that arm to keep my balance!
This crawling stuff is hard work.
Wanna try again, Mama?
Here we go!