If your air horns are open faced you may have a plug inside the horn its self. Check to see if your air lines are leaking using a little detergent and water in a spray bottle at all joints and anywhere the line may have rubbed against some thing. Some one will have to hold the horn button down once you get on the horn side of the solenoid valve, while checking for leaks. It takes pressure and volume to make them work One more thing, see if the valve to the horn solenoid is wide open. Hope this helps.
99 F-350 4X4 PSD pulling a 24' Nash travel trailer