The below issue occurred in rekordbox Ver.4.2.3
Please update rekordbox to this version (Ver.4.2.4).
-Sync Manager did not sync playlists.
When you sync playlists which were not synced in Ver.4.2.3,
firstly please untick the unsynced playlists and click the Sync button (the arrow icon).
Then, tick the unsynced playlists again and click the button to sync them.
*Other features added and fixed in rekordbox 4.2.3 work as expected. Please see the release note of 4.2.3.