
@Leo > the following could be the issues:
1) the cables; swap them with another set
2) the speakers; swap them around and see if the one that currently "doesn't work" works
3) L-R Balance; ensure you have it at 12 o'clock.
4) The mixer output; if you are using the RCA maste outs, try using the booth outs instead. If everything works correctly using the booth outs then it would appear you've a technical issue with the master out pots. In this case, you'd need to bring the unit to an authorised service centre for repair.