Hi all. I had a 9 month old start 3 weeks ago and I'm not sure whether to be concerned or not. He can't sit up by himself and can't crawl. He can roll over and that's it. I've tried to sit him up and he just flops over. All of the 9 month olds I know can crawl and pull theirselves up to stand. Is this normal or should I be concerned? Tia