two common things that can happen:
1) F&E short of water
(which you say it isn't, presumably you have pressed the ball down and water squirts in, like in a WC cistern? there should be no noticeable sign of water movement in the F&E at any other time)
2) blockage in F&E feed pipe which prevents water going down to refill the system (this is very very very common)
Is the bottom of the F&E thick with rusty mud?
If you bleed the highest radiator in the house, does water squirt out vigorously, with no drop in pressure as you vent it? What colour is the water?
(this is easier and quicker than drawing off a bucket of water from the drain cock and verifying that an equal amount of water is drawn down from the F&E)
I am just a householder but have experienced both these probs