Armitage shanks quiet float valve - fault ?

Armitage shanks quiet float valve seems to have a fault. Water continues to flow in (and down into the pan) when it should shut-off.
I've changed the rubber diaphragm, but the problem re-occurs. Doesn't happen conistently though. Have tried taking the diaphragm out and putting back in, tried a different one, problem still present.
Any suggestions ? !
Thanks