Problem with Channel 1 on Qu-32

I have the Qu-32. I have my computer on Channel 1. All of my channels are set up to be Pre Fader (Mains and monitors controlled separately). But for some reason I can’t get Channel 1 to be Pre Fader. I’ve tried pushing Assign + Sel button and nothing happens. Everytime I turn up the Fader for the mains, the monitors get louder also on this Channel only. Am I doing something wrong?

When you say all the channels are pre fader…

Each channel has a pre/post selection for each mix, possibly easiest to see on the routing screens?