No way to know without seeing it, but my guess is that they're all simple 'open/close' operations, and they all need to be open. I can't see why one would be closed in normal circumstances
try turning on the hot tap in the room where the tap is not working,when its turned on go to the hotpress and one at a time turn each of the 4 valves in the opposite direction .if the hot tap doesnt come on after turning the valves,return them to their original position.
well it was anti clockwise for open, i opened em all but there was only a trickle, ie an airlock, i put the garden hose spraying into it and it sorted it out, ta for all yer help