dont forget, some of us on here although classed as "septembryo's" had babies born earlier and later than when they were due, and therefore, what they are doing will be at different rates.
you know your child, if they are happy with what your doing then dont worry about what others are doing. if you feel your LO needs more tummy time then leave them an extra 5mins on floor, or sit and play with them longer etc.
i found my eldest to be the slowest out of all 3 (he's not slow at learning now and doesnt have difficulties apart from concentration), but this was because i had to amuse him and do stuff around housework etc. when middle son was born eldest was just under 3 and couldnt do enough to amuse his baby bro and therefore middle son learnt things much quicker. and the same with blake, he is copying the older 2 and therefore picking things up.
not all babies crawl, some go from sitting to walking straight away, some roll others dont, some sit on own and are happy to be in 1 place.
a lady at the kids school has twin boys (now 5yrs). 1 rolled around and 1 crawled, and she was worried that they wern't both doing what the other was. HV told ehr that all babies are different and not to worry. and her boys 1 talks non stop, the other is really quite and again she was worried that the 1 that didnt talk had problems and he wwent to speech therapy - and he wouldnt shut up!!! he just didnt say much at home because of his twin bro.