The clamp has a screw through it that can be tightened and loosened to help with height adjustment. Put the clamp flap in the upwards position, then using a Philips or flat head screwdriver. The clamp/ screw is REVERSE THREADED. Tou can tighten by turning LEFT/COUNTERCLOCKWISE, or loosen by turning RIGHT/CLOCKWISE.
Raise it to the height you want and then tighten the screw.
Once tightened, put the flap down and it will hold steady.
Watch the video demo below: