HELP! Using a mixer, only MASTER not CUE comes out of headphones

Hi! I bought a new DDJ400 in October and have had trouble since then with the headphones. The headphones/CUE feature work fine when I'm using headphones & playing out of the laptop / DDJ, or even when the controller is plugged into a speaker via RCA directly, but... not when I'm using a mixer via RCA cables (and have tried other people's headphones, same issue). The song CUE volume in the headphones is so low, like 10%, even when turned to maximum, so I can't use the headphones for mixing which is a big problem. I can hear song PREVIEW fine. The MASTER song plays loudly from the headphones even when the headphone dial is set to CUE only. I have the latest version of Rekordbox, using a macbook air Ventura 13.2, have Master Cue on etc, have checked the sound settings on Rekordbox... Please help, I cna't use my headphones for mixing parties and Pioneer says I need to ship off the device for 6-8 weeks for repairs, which obviously doesn't work with weekly gigs!