Hey There....
No you shouldnt be worried at all, babies all go at their own pace.

Maddison was crawling by now & my niece never crawled til after turning 1 & started walking 5 months after that, but is a perfectly normal 3 yr old...
Indah rolls onto her tummy coz she loves it & is happy to stay on her tummy, but Maddy hated her tummy so she rolled back onto her back & would then stay there happily!

All babies are different & do things when they want to, not because someone else is...

She is perfectly normal, happy, healthy & contented....

Your doing an awesome job keep it up!