I assume your using both Left and Right main outputs.
Do the main output meters show the same level?
Try using the ALT OUT outputs, you’ll need 1/4in TRS to XLR adapter cables. Check the ALT OUT is patched to the main L R outputs.
Have you tried swapping the cables, plug the left cable into the right and right into
the left and see what happens. If the problem stays with the same left speaker the issue
is in the cable or speaker, if it moves to the Right speaker, it’s something at the mixer.