Stereo Audio issue

I have my Flx4 connected to amplifier via RCA cable . Suddenly the left speaker stop working unless I turn the amplifier mode to Mono. I checked cables and connectors and everything working well . Rekordbox Master audio output is FLX4WASAPI. I can' find any other option I can make any adjustment. even tough the issue seems to be at Rekordbox setting level.  I'd appreciate any help on this. thank you