...so I've been asked this question a few times now...In one student's words: "Is it normal to lose poses?"
...by which she means: is it normal for your expression of a pose to look one way for a while and suddenly one day, it's different? Yes, yes, and yes.
There has yet to be anything static in my practice and I suspect the same holds true for others. While a pose is never the same twice, some changes are more noticeable than others. As a natural backbender, I took my heels without much effort on my first try in Kapotasana. Initially, it was not a pose that I struggled with (although there were and are plenty that I struggle with daily). Some time after, when my teacher introduced the various leg-behind-head poses of second, I lost Kapo. It was puzzling. One day it was there and then one day it was suddenly a crazy intense pose and even my toes seemed very far away. As I wound my way through the experience, over months, Kapo to heels returned, but it's different.
If there was one thing that seemed much clearer to me during the months of that particular transition, it was that the act of doing the pose and noticing whatever was going on was "yoga". The expression of a pose, what it looked like on any particular day, was really neither here nor there. As it has been quoted around the cyber shala: The particular expression of a pose "doesn't last, doesn't satisfy, and isn't me" (-Daniel Ingram I believe; someone correct me if I've gotten that wrong.)
I love Kaivalya's description of a personal body gremlin who moves around hanging out in one place in the body today and settling into a new spot tomorrow. It's such perfect description of the endless little openings and shifts that occur in the body.
The ongoing shifts in my physical practice play back into my mental practice, never letting me get too comfortable, too attached or inattentive in a pose...because as soon as I do, it changes.