Official comment
Probably a software issue - try this (if you're on a PC, skip to step 7):
- Open the Applications > Utilities > Audio MIDI Setup app
- Click Window > Show MIDI Studio
- At the top of the window, click the drop-down menu and select New Configuration...
- Give it a name (eg. 2023) and click OK
- Click the IAC Driver (should be red) and press the Delete key on your keyboard
- Close the Audio MIDI Setup application
- Open Serato DJ Pro
- Open the preferences
- Click the MIDI tab
- Click the DDJ-SR2 in the upper window
- Un-check the Allow Serato Hardware Remapping button above that window
- Click the Restore Defaults button at the bottom of that window
- Close preferences and test as usual