Form checks and butterfly stretch question

Hi,

I started the MS series a few weeks ago and I think I'm doing ok on most stretches form wise.  The one I can't do properly is the butterfly stretch and I'm trying to understand why. Despite elevating myself I can't flatten my back. As soon as I pull my feets to the groin the lower back is pushed into flexion and rounds. Elevating the butt more just puts too much pressure on my ankles and feets. The only way for me to feel a good stretch in the groin is to lean back and push the pelvis forward using my arms. Anyone that can help me understand where my limitiation/restriction is?

Along with the butterfly stretch variations I added some pics to show my form on some of the other stretches. Feedback is much appreciated!

Thanks!

Butterfly stretch attempt:

Butterfly stretch variation:

Floating pancake:

Diamond frog:

Piriformis R:

Piriformis L:

IN this case, the capability to make your lower back flat is reduced from the posterior hips capsule. My suggestion is having a check from a PT, then if this is the problem I can give you something more specific.

Ok thanks @Alessandro Mainente, I'll have it checked.

Is the form ok on the other stretches? 

Thanks!

yes they are ok.

Great. Thanks for helping @Alessandro Mainente.