When does one move on from a pose?

A comment I received on my last entry made me wonder: when DOES a teacher move one on from a pose? I always thought that tradition has it that one does not move on until one has mastered the pose one is working on. But then, what is mastery? I have noticed that many people who are working far into Primary and even in Intermediate still seem to struggle with certain poses in the Standing Series. Are there certain poses that are less important to master before moving on?

For example, I see a lot of struggle in Ardha Baddha Padmotannasana, even with practitioners who are well past that in their practice. I also see students who are not even close to "completing" Janu Sirsasana C(i.e. the Manolo Blahnik Stiletto Foot Pose). And it seems like the teachers I have had so far do not give a lot of adjustments in Janu C (looks a bit scary to go there, at least to me...). In my case, Parvrtta Parsvakonasana is not quite there yet - my bottom fingertips are just grazing the floor now. I do note that this pose has not always been IN the Standing Series - it is not included in SKPJ's Yoga Mala.

So the question is: Is there a MINIMUM level of competence for each pose?

Then also, in my case, I have been permitted to move past Mari D despite needing help to bind every single time I practice - presumably because the poses that follow (Kurmasana and Supta Kurmasana) will help me to soften the scar tissue along my chest and shoulders. But that is a special case. But then, aren't we all unique and special cases?

Also, what happens if one day you are practicing and you just can't bind in Mari C, and the teacher is not available to help you? Do you stay there and wait? Do you move on? Do you just give up and move on to backbending? Similarly, in Supta Kurmasana, I have noticed that at AYS, the teacher always places me (and most others I see) into the pose from Kurmasana. But is that true everywhere? Am I supposed to be learning how to get into it myself? Am I supposed to be learning how to pick myself up from the prone position into the vertical arm balance transition to Tittibasana?

So many questions.

YC