Hey there.
I just wanted to share my recent experience. Maybe it could help someone. Since the beginning of my middle split work i always felt limited more from my right leg than from my left leg. I thought, if this right leg would not restrict me, i could go deeper into the stretch. And, impatient and nerved as i was, i pushed it too hard, until i felt a short, sharp pain within the hollow of this knee, which has restricted me. I paused, long enough. But always when i came back, i felt like i could not make any progress, because my right leg restricted me still.
And then, some day, angry at first about my right leg as always, i tried to calm down and massaged my leg... Until i got a flash of genius. Foolish as i was, i had never thought of this possibility before. The reason of my pain, and the reason of my restriction was simply: I always had my right leg a little bit further to the side (in relation to my hip) than my left leg. I didn't realized this ever, because i just stretched both legs out as wide as i could. But evidently, the range of motion of my right leg was better. So it was impossible for the pressure of the stretch to be equally on both legs. I needed only this slight adjustment and i felt the stretch in my left and right leg equally, as i wanted it from the beginning. And two weeks later, i had made more (and clearly visible) progress in my pancake than by several months of work before.
Since then, during the pancake work, i put my feet on one line (through markings on the floor for example or a stick in front of me), then check the form of my hip. For sure, i don't know if anyone is that foolish as i am, but if you ever experience such pain in one leg only; consider if your legs are equally stretched out to the side.
Be consistent.