The problem may not be in your feet...exactly. Keeping a horn steady on your face while moving around a field is not a natural thing for your body to be doing. In order to get this task done, your body can do one of several things. You could be holding a lot of tension in the arm/shoulder area, which translates into pressure on your chops. So if sound fine standing still and have problems keeping the sound up while you're moving, this could be the problem. To correct this, think of stabilizing yourself from the abdominal region (this is also known as the marching "powerhouse" or "core"). Proper posture and a minimum of tension are the best remedies for failing chops.