Official comment
Please try resetting the MIDI by opening rekordbox, clicking the MIDI button at top right, then click the DEFAULTS button. You will be prompted if you want to save your current configuration - if you have made any customizations, go ahead and save it, otherwise just go ahead and restore the defaults to see if that helps. If not, it may be a hardware issue , in that case please contact Pioneer DJ technical support in your region to have your DDJ inspected and/or serviced.