Check your control head.
It's the control lines, or more accurately the trigger control that is operated on air pressure. It might be leaking at the control head or a crack in the line (it's the thinest line in the bundle). Also there should be a pod at the base of the boom that this line plugs into. There is a air intake filter in there that clogs sometimes. Is a very low pressure sort of thing. The pod generates air pressure in the control head when you compress the trigger. With low or now pressure the low pressure control valves in the control head won't work or will work slowly. This is providing that you have a pistol grip control head.
I also could be cracked control lines at the boom elbow. Or maybe your control head valves are so worn that they are sucking air but then the control head would be visibly leaking a lot, I would think this would be a gradual degradation, valves working slower and slower until they stop working. The fact that you have full control at the dead-man indicates that it's not in the main hydraulic, high-pressure valves or lines.
Ya, check the pod, make sure it's pumping air, that's you're best and cheapest bet.