Work smaller areas and slower arm movement to let the polisher do the work. You are consistently lifting the handle end of the polisher forcing the front down into the paint. I know you think you are keeping it flat, we all do. Flat front to back and also side to side. X and Y axis.
I have a buddy that helps me sometimes and its a struggle to get him to slow down his arms. He thinks he is adding to the polisher's movement. He never seems to work the polish evenly because of all the movement.