Behriger Europower PMP1280S low volume from output B

[zor]

In stereo mode, output B has low volume and output A is fine. In mono mode, output B has low volume from the Main side and output A is fine using the monitor side. I switched the wires to the connectors A & B, the problem followed the wires so it isn't the connector. Seems I have a problem with the B side. Any suggestions would be great