One vertical labyrinth-type, 2-stage reciprocating compressor in our unit has failed, and we are seeking help

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Let me first provide some background: the unit features a Sulzer reciprocating two-stage labyrinth compressor that was installed when the facility came online in 1998. The problem right now is that when the unit is started, it loads 50% automatically and then 100% manually; however, there is no change in the sound of the unit, so it seems like no loading is taking place. The traffic is significantly lower than normal. The second issue is an overreporting of the primary outlet pressure. The secondary intake and primary exhaust valves have been checked; the leak test results were satisfactory. The heat exchanger between the primary and secondary stages was also inspected, and no blockages were found. The unloading valve was checked and is normal. The primary and secondary pressure tap wires were checked and are normal. It is currently suspected that when loading manually, the gauge may not be sending a signal to the unloading valve to cause it to operate. I would like to ask the experts what other possible reasons there might be.