@Tomislav > A couple of things to try for troubleshooting:
Does the the master does not show signal on the LEFT side? If not, does it output audio on the LEFT side?
Connect the source (CDJ or turntable or whatever it is) to another channel and see if it comes out the master output on both L and R
Put it back in CH2, but reverse the audio so the white connector goes to R instead of L and connect the red to the L instead of R; if it's not working, you should have the audio reversed and you should still be missing the LEFT audio from the master, although the left signal from the player will now be coming out the right speaker.
Switch the master balance switch between STEREO and MONO; if it only comes out the RIGHT side in MONO mode, then the problem is possibly the output (whatever is connected after the master output), but if it comes out in both speakers, the problem is possibly the channel 2 fader.