Lets think about for a second.
It COULD definitely be any/all of the above mentioned muscles without a doubt. Or it could be something else entirely. I recommend that you do to things to find your answer. My suggestion is take a real good look at it, maybe even compare what you see to others that can perform seated in that position the “ideal” way. If nothing else to get clues. Maybe you will see that the pelvis isn’t completely scooping underneath or that the legs aren’t getting fully extended. You may also see that the lumbar is flexed and the t-spine is kyphotic. So simply, make an observation.
Second, and really the easiest thing to do is ASK. Ask your client, what they feel and what it is that is impeding them.
Putting an observation together with verbal feedback should start you down the road to figuring out what corrective approach you can take to get this “fixed.”
hope this helps,