If it is a ball valve you can adjust a plastic screw to allow the cistern to fill up to the required level.
see pic here:
http://www.ultimatehandyman.co.uk/ball_valves/diaphragm_ball_valve_adjust.jpg
ther is usually a line inside the wall of the cistern for the required level. when it is at the required level it should flush properly.
tony
April 2010