Trouble with YoYo game

Yesterday I went to the yard before work to have a quick play with Filly. ( By the way I'm writing this from Philadelphia, which is kind of appropriate !) I still wanted to improve her tendency to nip so concentrated initially on the "stick to me" game. She was much better today. Whenever her head came round at me with an open mouth I only had to move the carrot stick towards her for her to think better of it. We also did lots of turns towards her as forequarter yields are generally what triggers a mouthing attack. Again much improvement on previous days.
Using forequarter yields are one of the ways that horses determine the pecking order in the herd. I am sure that many of you have seen the fencing (as in swordsmanship) games that horse play with their necks and teeth. They rarely actually bite each other (though one horse on the yard currently has evidence that they do occasionally) but use their heads almost like clubs trying to knock the other horse away. The looser is the one who yields their head or ultimately their front legs away. This is in effect what I am doing as I ask for a forequarter yield. The important thing in this game is to never move your feet away from the horse. Towards is OK as soon as they start to yield so that you can follow them and eventually get a full 360 degree turn with the hind legs remaining in one place. Alright, I can dream can't I !! It may happen one day.

I think she will always be a horse that tests the boundaries of what is acceptable. In a way it is this spirit that I find fun. You can never assume anything with Filly, it doesn't change on a daily basis, but on a second to second basis. It can be a bit frustrating at times I admit, but the reward is enormous when it works well.

We then moved onto the YoYo game. She is very good at the draw ( towards me ) but the backup is to put it politely slow. I try to keep the phases consistent, starting with a finger wiggle, moving to a rope wiggle, a wiggle that moves the halter on her nose and finally a full hard rope wiggle with lots of commotion. She does go backwards, even at phase one, but you could go and make a cup of tea in the time it takes to move one leg, and I want her to go back 22 feet. A full English cream tea could be consumed in that time. I have tried making it more interesting for her by backing between cones, over poles etc, but it is still slow. Today I tried making the phases quicker, which caused her to look interested, not at all alarmed, even at phase 4, but move at about the same pace. Maybe a tad quicker, but it was certainly me putting in most of the effort. After a slightly more acceptable backup I called it quits for the day and took her outside to apply more hoof putty.
I noticed that she was walking a bit lame on the concrete and went to pick out her hooves to find the cause. To my horror I found two round stones which she must have picked up between the field and the school ( I had picked hooves out in the field entrance) lodged in her bad hoof sole. The bar shoe she has on along with tiny feet mean that stuff easily gets stuck there. No wonder she was having trouble moving quickly, but she had not appeared lame in the school, and her expression had not revealed any pain. As Ritchie commented, "always check for a physical problem before assuming a psychological problem ".
 Lesson learned, experience gained, but a bad feeling of guilt all the way across the Atlantic !