I began typing this in another thread, but didn't want to hijack it cause I don't have a problem as such...
My DD (nearly 8m) can sit well and will sit and play happily for ages. On her tummy, she can get onto hands and knees and rock. She moves backwards (but not by crawling, but sliding). She has good leg movement, but doesn't get that she could move her hands at all!
She has been doing this for nearly six weeks now, and the moment she started people began saying "oooh - give her a week, and she'll be crawling" I am bored with hearing it now. However, she gets extremely upset and frustrated at her inability to move forward, and none of the "put toys just out of her reach" and "frustration is the key to getting them to crawl" advice seems to help. She just cries and it upsets us both too much to just let her stay there. She is fine once I pick her up and sit her down, or stand her on my knee and let her bounce up and down.
She'll just suddenly get the knack one day right? So, I'll just keep popping her on the floor to "practice" every now and then? I know this is a daft post - but sometimes you just need to hear other people's experiences!