DDJ-SX2 with Rekordbox 5.0.1 - Deck 2 won't scratch during playback

This problem started a few weeks ago and I can't figure it out.

Decks 1, 3 and 4 all let me scratch during playback but Deck 2 won't. Everything works as you'd expect when Deck 2 isn't playing but as soon as I hit play the deck won't scratch, only slow down or speed up the tempo. Seems to be a software problem or a setting I can't locate as all other decks scratch fine. Any idea how I can fix this please?

Thanks