Baby still not rolling

My baby is 5.5 months old, ca. 8 kg (sweet chunky girl). When she was 4 months old, I've noticed during tummy time she's trying to roll - so exciting!

However, the weeks have passed and no more attempts to roll were made. I tried to help her, showed her how, did all the exercises I found on YouTube - all for nothing.

She doesn't particularly enjoy TT, but we do it regularly. Usually she just works her muscles out for a moment, interested in all the toys, doing swimmers, until a minute or two later she just gives up, puts her hand in her mouth, and lies down like a pancake on the playmat. No matter what I try, she becomes completely uninterested in anything and soon starts screaming.

A few days ago I've noticed whenever she's in her baby gym (on her back), she's doing a half roll - lying on her side and playing this way, waving her legs in all directions. She's also doing a ton of baby crunches as if she tried to sit up this way.

In general she's happy, talkative, likes to eat and take a bath. She used to have a terrible reflux, but it's much better now. She keeps her head strong, is extremely interested in the surroundings and loves strolls.

But still, no rolling, which makes me super concerned about her development.

Any advice?