Eureka! It was brought to my attention that I’m shifting my weight to the outside of the 20 meter circle! This is causing my horse to step under me (think of leg yielding) and out of the circle. Also I’m still not using the inside rein properly. I need to adjust my inside hand so that my fingernails are up and my thumb points toward the center of the circle (hitch-hiking). The minute I do that my horse is coming down and nicely rounded. This positioning can be used when riding corners too.