not crawling

My son will be 9 months on the 19th and is showing no signs of starting to crawl.. when he is on his belly he just lays there and moves his arms and kicks his feet. He will only lay there for a short time and then start to cry. My husbands family keeps making comments about he should be crawling by now and it's frustrating me.. any other mommas have babies this age that aren't crawling?