Is the cistern filling up to the mark on the inside?
if not the valve needs adjusting to let more water into the cistern. At the water inlet end of the ball valve assuming it's the plastic type there is a plastic lock nut on a plastic screw plunger which shuts the water off when the tank is at the set level. Undo the nut and turn the plastic screw anti clockwise [towards the ball float ] do it about a turn and half at a time and stay with it to check the level the water is when it shuts it off. If it's still to low then turn it some more. When you get the tank full enough to flush away waste then re-tighten the lock nut.
DIY'er, March 2005