Dressage lesson - possible explanation for Aspen?

Maybe the fact that I'm struggling to translate (and then practice) my lessons are part of what the problem is at the shows.  We're riding at the highest level I've ever ridden at, on a horse who is sweet and willing, but who hasn't been trained to this level by a pro, and who isn't a natural dressage mover.
Maybe the fact that every lesson is a stretch of my brain (in a good way), where we get, at the end, some absolutely beautiful and amazing work I never would have thought we were capable of - but I can't repeat it on my own.  I can get a movement, here and there, and over the summer, in our baby steps, we have improved dramatically, but it is taking tons and tons of repetition and practice and work and time with John before I can start to apply it myself.  I'm just not naturally talented.
But John was disappointed.  He said we looked fine in warm up, what happened in the arena.  And I said nothing - it felt the same to me.
So John told me to tell him when I felt like Duke was as round as he was during his test, and then he worked on making him rounder.
What we did was work on a 20/15/10 meter circle around John, and we would use counter flex, haunches in, leg at the girth, hand on the neck, elbow on my hip, etc. to fix whatever Duke was doing at that moment to avoid being round and springy.  John said that even with him riding, Duke just doesn't have enough muscle on his topline to spring around like a warmblood the whole test.  To avoid the bend, Duke would pop his head up (or his shoulder out) and my job is to - as soon as he starts to do that - put leg on to push him forward so he doesn't.  Then when goes round again, give a little.
John helped us set it up better than I do.  Like he would get him bent to the outside first, and then ask him to go forward.  And when he started to fade, John wouldn't let him get away with just shuffling around for a few circles, he had to go back to work pronto.
Trying to describe it is hard, because it is ... John has me react to each step, each movement, and so it is constant adjustment.  And the constant tiny adjustments make Duke look like he's being consistent.  Or feel consistent.  Every once in a while, we would get all the stars aligned, and Duke would just feel amazing - balanced - but even his "unbalanced" feels fine and good to me, so it's hard to retain the "best" feeling instead of the "good" feeling.
It was a great lesson, and I'm continually amazed that the two of us together can ride that well.  But I'm sorry we disappointed John (and that he's stumped) and that I don't know what to do different because I'm not sure what I'm doing WRONG at the show.
But like I was saying to John, man, we are so much better together than we were six months ago, or two years ago.  He's not even close to the same horse, and my posture and seat have improved.  Our dressage scores might not have - and I know that's the objective measure, but subjectively, I'm 99% confident that we are much, much better than we were.  At one point, John had me push him out to a bigger circle on the canter using my inside hip bone.  And Duke did it.  Immediately.  It was divine.