DDJ-GRV6 – Decks 3 & 4 play but no sound from master output

My DDJ-GRV6 isn't outputting any sound from decks 3 and 4. Decks 1 and 2 play normally. 

Tracks load and play on decks 3 and 4 (waveforms moving), and the channel 3 and 4 level meters in rekordbox are moving, but most of the time nothing comes out of the master output. Or it plays but distorted. 

I've used 2 laptops and have the same issue. 

I'm using Rekordbox and don't have this issue with another controller.