This typically indicates a problem in the air ride system.

There is a leveling switch on the axles that control the valve to level the air ride. You may have to disconnect the linkage to allow the bags to fill.

It may indicate foreign materials in the air system, e.g. material in glad hands or the air lines control valve, etc.

You may have to assist the air ride by lifting the unit with the rear hydraulic legs, allowing the bags to fill. DO NOT raise the unit too high or the leveling valve will sense it needs to lower the machine and just exhaust the air.