I just made a video showing deck 3 not outputting to the mains but then I thought I’d keep playing around with the various buttons on the SB2 and what I discovered is that when I first select the deck 3 button I also need to press the pad trans button (toggle it on/off). At that point deck 3 output starts going through the main output. This is the same for deck 4. Once I’ve done the pad trans toggle once the deck buttons work normally the for subsequent tracks with output for all decks going out the mains.
BTW, I saw this same issue using Serato DJ Intro so this issue is not specific to Djay. I wonder if anyone else with a SB2 has seen the same problem? I ask because it may be that my deck is defective.