I assume you installed it! if so and the cistern is fed from a tank in the attic you may have a mains pressure (high) valve in the ball valve instead of a tank fed (low) which can make a difference. the other thing is if there is a stop valve or an inline check valve, the latter is the ones you use a screwdriver to open and close, may not be open fully so flow is being restricted. If my memory serves me correctly the low pressure flow valve is white and the high pressure one [mains pressure] is red.
Have a browse around your local DIY Store and you may find one pack open and you can check the fittings.
DIY'er, March 2005
if the water pressure is fine then it is probably a blockage in the ballvalve, take it apart and see if there is and muck in it
