Some users have reported issues after upgrading to iTunes 12.2 and attempting to use the sync to rekordbox. Our rekordbox team has found that the XML behaviour of iTunes has changed, and as a result, the playlist tree was not updating properly within rekordbox.
To resolve this issue, please open iTunes and open the Preferences. Click Advanced and select Share iTunes Library XML with other applications.
Should you have any further issues, please post a question in the User Forums.