DJM A9 + rekordbox dvs - decks not playing

I have a set of 4 rather old cdj-s with RB control CDs in and a djm a9 mixer, as interface. I connected everything as should be, set up RB accordingly, but there is only one deck playing - Deck 3 (CH1). All 4 channels get the signal from the CD and there seem to be no obvious reason as to why they shouldn´t be playing. Am I missing something? A setting, or procedure? I´m kind of lost here.