My 8 mth old son.. will not sleep, sit up, or crawl!!! Ferber Method??

i don't know re sleeping i don't have a problem there but my girl (8 months) will sit but will not crawl and has only just started going on her tummy she refused before don't worry there all different develop differently i worried but saw other mums and babies all doing and not doing different things. with sleeping i let her cry out for 5 Min's go in and sooth no picking up just stroking and shushing when calm come away the next time 10 mins ect ect. i did this a long time ago and she leart quickly mummy is crap at night. but also girls do develop faster than boys xx