Why is Emilia not rolling over

Ever since the neonatal follow up clinic, I have been obsessed with "why" Emilia is not rolling over yet. I know babies develop and grow at their own individual pace and not all babies will roll at the same age but the follow up clinic left me feeling like Emilia is supposed to be able to roll over by now. She's 4 months but she's not rolling on her own yet. I need to hold her wrist and give it a gentle shake (a bit of a nudge) then she will lift her two legs and pelvis to cross over and roll. Her physiotherapist tells me that lots of babies these days don't seem to roll... some of them just skip the whole rolling and jump to sitting and crawling. She thinks it's because of the Back-to-Sleep campaign. Well, whatever the reason, I feel it's my fault that Emilia is not rolling over yet. I have been doing lots of exercises and tummy time with her since the follow up clinic. I dearly hope she can roll by the end of this month. She has a checkup appointment with her pediatrician at the end of January and I would hate to say "no" again when he asks me if she has started rolling yet.