During my lessons, when I'm practicing serves, my coach tells me to bend my knees, which I do, for a little while. Then I begin to forget and I just stand there. It's not just when I serve either. When I'm hitting groundstrokes my coach constantly has to tell my to bend my knees and stay down throughout my strokes. Why is this so hard??? I always seem to be popping up, and as a result, the balls are always going out. If my coach wasn't there to remind me to bend my knees, I probably would never do it.