Why wont the airbags on the heay trailer lower?

there is a valve, on newer trucks it's most likely elec over air. when you activate the drop valve it sends electricity to a selonoid (usually on the trailer) which in turn operates a valve to open and release air from bags through exhaust ports on that valve. Some valves use a diaphragm gasket type, other valves use a round dowl shaft with o rings and slipper seals. The diaphragm type could possible have a ruptured gasket,or selonoid is bad. The other valve could have bad 0 rings, or have debris in cylinder which has it stuck in the up or fill position. remove air or electric from these valves or valve and check for power or air flow. also there are control ride valves on the trailer that work off the axle that tell the air system to fill and exhaust for suspension. the valves consist of plastic internal parts which could fail.