Air compressor not blowdown

Hi, Please, we have a 20hp Gardner Denver rotary screw compressor for air production in oil facility but of recent we started have blow down problems. This is the scenario: The unload pressure is set at 135PSI and load pressure at 95PSI. Compressor is programmed on AUTOMATIC MODE and it builds up pressure to 135PSI, unload properly, blow down but not being able to depressurize the oil reservoir, and as a result, the compressor runs continuously even though it is on AUTO MODE. Any info on how to resolve this problem. Thank you.