Always check the phono/line level switch when jacking into a club's mixer.
Always make sure it's set to stereo, not mono.
Always make sure that both your left and right cables are securely plugged in.
The first 2 are recent. The 3rd is something I did a few months back - 20 minutes into my set and realised what was wrong : thought the booth system was just busted on the right channel.
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