One thing that you could do is try some bareback work with the horse you have the trouble with. Another thing I heard about is tying the stirrups to your girth with zip ties or twine so that your leg is forced to stay by the girth. I have not tried the tying method but I have started to do bareback and no stirrup work and it has helped a lot.
Hope this helps :)
I keep my heels down and have a strong leg, I just don't know why I can't