The ballcock is set too low and there is not sufficient water in the cistern to make it flush. If it is a metal (brass) bar, it can be adjusted by bending the arm higher to allow more water to fill the cistern, before it blocks the flow. If it is all plastic, the will be either a screw where the plastic arm joints the inflow pipe, or an upside-down (egg-cup sized) cup which can be raised higher on the notched piece of plastic going through the centre.
Iain, November 2009
