You can rotate them, however the flagella automatically teleport to the nearest membrane edge, and from there a line is drawn to the center of the cell. And the angle between that line and straight vertical line, determines how much thrust the flagella generate. So for example if you place a flagellum directly at the back of your cell (and your membrane shape ends up being circular enough to not displace the flagella away from directly behind), then that flagellum has 100% of thrust available. Instead if it’s slightly pointing to the side, you get much lower percentage of thrust.