Definitely don’t worry…are they moving their body a bit more when on back? Ie, mines not rolling but lifting his hips and legs more , can roll on certain surfaces if he’s on his front. My first son did it more like 5-6 months as well.
They definitely run their own races! So don't fret. If it helps, both my babies I encouraged to roll using this same technique in this video 😊 https://youtu.be/NCBEYie42iU?si=8Vr1DU-bHygNhPLW
Mine isn’t yet but is tracking well with everything else
Mine isn’t rolling yet either. Lifts legs and sometimes overbalances but nothing intentional from the looks of it
They’ll roll in their own time. It’s still completely normal not to be rolling at 5months!