where is stiff if I can't sit deep enough at side-to-side squat?

Just like the title, what is the specific reason if I can't sit deep enough at StS?

I ignored this at PE2, but now it hinders me. 

I want to know anatomic names of muscles, tendons which prevents me from doing this correct.

I already posted to get better ROM(link below including video), got some advice but still need more knowledge.

https://www.gymnasticbodies.com/forum/topic/20059-need-help-hip-joint-rom-in-side-to-side-squat/

Mats Trane advised me to put an arch in lower back and keep APT before sit down but it's hard.

I couldn't keep APT and straight back. It breaks as soon as I started to sink.

so I'm looking for alternatives to increase my ROM. 

by the way, I'm doing all stretch courses once a week. would that be enough?

Depends but it could easily be hip and ankle mobility, can you drop into a full squat without falling over backwards? - practice your third world squat for 10 minutes every day.

Your inner thigh and hamstring, S:FS will address this over time.

Or, poor movement pattern. This will improve as you practice StS, you can try holding onto something as you practice.

MSs/SE4 corresponds to your issue. I'd recommend you post a form check in that forum.

Murray Truelove said:

Depends but it could easily be hip and ankle mobility, can you drop into a full squat without falling over backwards? - practice your third world squat for 10 minutes every day.

Your inner thigh and hamstring, S:FS will address this over time.

Or, poor movement pattern. This will improve as you practice StS, you can try holding onto something as you practice.

Oh, never herd of third-world squat so I searched it on google.

It is no more than natural sitting-resting position and I could easily do this. 

but as my feet is apart more than shoulder width I cannot sit down, hips are lifted.

well, in StS feet look twice as apart as the athlete's shoulder width, did I saw wrong?

should I practice wide version of third-world squat than? 

and what do you think of Mats advice? He told me to set an arch in my lower back and APT before sit down.

but third-world squat is bent lower back and PPT, just the opposite.

Cole Dano said:

MSs/SE4 corresponds to your issue. I'd recommend you post a form check in that forum.

MSs/SE4! I couldn't do this properly too. thanks I'll work on it and post the video on Saturday.

please leave some comment.