There are three things that may be going on here: (1) your instructor is the type to try and pick one or two things to correct before moving on, strategically or (2) your instructor doesn't have the ability or experience in teaching to progress you and your daughter as desired. Have you seen the more advanced students of this instructor ride? What level/ability do they possess? How long have they been there? or (3) this instructor's teaching method just may not be for you.
I've had an instructor that was a combination of 1 and 3. I will say I learned a lot in my time there, but there came a time where I outgrew the barn and branched out to instructors who specialized in the discipline I chose. I will also say that I do prefer an instructor who is constantly critiquing me and pushing for progress, which is not what everyone prefers. At the same barn, there are many instructors I would put in category (2). These instructors are looking to get their certification and are "shadowing" under the head instructor. They are inexperienced in teaching, and thus less flexible on adjusting their teaching to different riders, although some are still very talented at teaching anyways. Some, I would also argue, just don't have the extended riding knowledge to correct and foresee rider error since they still ride at the lower levels too.
I'd ask you the same question as Tinyliny, "what experience does your instructor have in teaching?" and "what experience does your instructor have in riding?". Your instructor does seem young, but that doesn't necessarily mean she doesn't have the experience. There are many very talented riders in high school.
If it were me, as it was at one point, I would move on when I felt I wasn't learning much at all after a series of lessons. It's true that progress doesn't always come in leaps and bounds, but if it has become a pattern, then I'd say there is little chance of it changing. You also have to take into account how often you are riding. Progress comes much much slower when you are riding 1x a week. Some instructors are quite talented in getting concepts across;however, correct position takes a degree of muscle memory to maintain and that is harder to develop at 1x per week.