Rolling over??

Brooke
Anyone else's baby not really interested in rolling over? Mine rolled both ways at about 3 months but hasn't really rolled much since. Instead she gets really grumpy until someone rolls her! 
She is sitting up and loves tummy time but is just too lazy to do it herself. 
I'm worried if I keep rolling her over she won't learn how to do it herself and won't develop properly?
Anyone have any advice?