Hi Anonymus,
What's happened if the driver blade has stopped in mid stroke, the air will escape. Quickest was 'round this is to take the airline off the tool, then find some scrap wood, place the tool on it as though you're firing it, and in THIS ORDER... pull the trigger and re-connect the airline at the same time. This usually sorts this out. Regardless of the tool make and model this will happen to most users sometime....
Leigh Co Limited