A couple of thoughts:
I recommend Bill Dorrance's book, "True Horsemanship Through Feel". Bill talks about the way of yielding the hindquarters that connects to the future task of hindquarter yield horseback, vs just demanding the yield, which'd be pressure where your leg'd be rather than the flank.
Also, perhaps Filly needs to find meaning in the request; it seems like now, she's saying, "what earthly reason do you have in asking me to move my hind?" Bill talks about connecting the task to the horse's mind.
Of course, I highly recommend the entire book! My theory is that "feel" is an invitation to the horse before one starts to pressure. If you can get it done by the invitation alone, of course, you would.
Thanks Northern. I've been meaning to read that book, but heard it is really difficult read and wanted to wait until I had enough knowledge to understand it. Maybe I do now.
As for the purpose, it's funny you should mention that. Now I have a softer feel to the yields I have been playing with purpose. For example I have been placing a pole on the ground and getting her to straddle it with one front leg and one hind leg either side of it using just steady pressure from my fingers. This as an exercise we had done with Bonitao until the point we could do it ridden as well.
It is good for the human and the horse. It teaches the human great timing with releasing the pressure. Too soon and the foot does not cross over the pole, too late and both do
It gives the horse a purpose to the exercise (not quite true they don't know why we want a foot each side, but the task gives us purpose that translates in our body language to giving them purpose), and a defined end point to the exercise. As an added benefit it gets the horse used to having an object under them which helps when out hacking and crossing logs, or jumping in the school.
I do other such purpose driven exercises as well, but that is an easy one to describe in writing.