Bad back flexibility

Hey GB forum! 

The pictures below are my gf max ROM before exercise. We've been doing once a week middle/frontsplit and bridge routine close to a year. She can get pretty deep in Jefferson curl though. And hamstring flexibility has improved, but the back hasn't. 
Do you know any suggestions how to improve 'chronic' flat lower back -issue? She has pretty flat back. 

Do you have suggestions how improve the back? She's pretty good in bridge flexibility, but the other way really is bad
Help appreciated, thank you.

Hello Sam, is she a powerlifter or CrossFitter?

This is how I feel. (I know, I should do my own form checks too.) But I'm curious what is suggested for her.

Luike, I'm a weightlifter and CrossFitter with a similar flat lower back in these positions. Plus, I really struggle to fully hit hollow position with the lower back flat against the ground.

Hi. No she's not. Never been to any kind of power lifting or gym. 

2 hours ago, Jennifer Marie said:

Plus, I really struggle to fully hit hollow position with the lower back flat against the ground.

This can come down to lack of core strength. Form checks would be necessary to know for sure. 

4 hours ago, Samu Riihela said:

And hamstring flexibility has improved, but the back hasn't. 
Do you know any suggestions how to improve 'chronic' flat lower back -issue?

Actually, Samu, a flat back/Anterior Pelvic Tilt, is ideal for pancake work. Her pancake will improve as the hips loosen. How is her piriformis stretch from the MS series? Can you send images of that position as well?

Ok. But for example when doing basic forward roll she can't do it rounded because the lower back simply stays straight like a wood board. 

Hi Samu, 

So her spine mobility is needing improvement. In Coach's Foundation One course he addresses Jefferson curls which are the perfect exercise in aiding spinal movement.