5.5 months - not rolling

Jenna

My girl was born Aug 20. She is not rolling yet. She went through a period of rolling tummy to back around 4 months (actually did it the first time at 6 weeks) but has stopped. She rolls from back to side and sleeps on her side a lot. Just wondering if I should be worried since it seems she should be mastering this at this age, or at least coming close. Thanks for advice