Channels 1 & 3 not playing through PA

The other night at my gig. Channels 1 & 3 on my mcx 8000 would not play through the pa, I was able to hear them in the headphones and get a level reader. Soon as brought the x fader the left side, nothing. I was able to use channels 2 & 4, putting channel 4 on the left side, so I could have a smooth transition… more or less . Anyone know what could of caused this? Thank you for your help.

Mac Mini Serato

If you put the crossfader in the middle of its travel, can you hear all the channels?

If yes, check the crossfader assignment switches for channels 1 and 3.

If using all 4 channels on the MCX8000 through some DJ software, rather than in standalone mode, then its worth selecting a different midi map/audio mapping on the DJ software for a few seconds, and then re-loading the original audio mapping - if still no luck on this route, then download the audio/midi map from the web, and try that - it could be that the audio map/midi map has become glitched within the DJ software/laptop.