Unless someone is pulling my 'plonker'.
I have been told but am not sure of this but each ball valve is supplied with two washer plungers, one red and one blue. One is for mains fed the other for tank fed.
Now don't quote me on this but you may have the wrong plunger in the valve [what the float arm pushes against to close off the water, it pushes agains the washer]. If yours did come with another plunger then change it over. Turn off the water undo the float arm undo the large knurlled nut on the valve remove the plunger and fit the other one see what happens.
Charlie
May 2009