I've started rekordbox CloudDirectPlay on my CDJ-3000X, but sometimes the browse screen doesn't display tracks uploaded to the cloud, or I can't load tracks. Why not?

There are 4 possible causes as follows:

(1) Cloud Sync is incomplete.
In rekordbox for iOS/Android/Mac/Windows, if cloud library synchronization and CloudDirectPlay ON settings haven't been completed, the track won't be displayed on the browse screen. Wait for them to finish, then the track should display.

(2) The track hasn't been uploaded to Dropbox or Google Drive.
Only tracks that you've uploaded to Dropbox or Google Drive, or tracks from streaming services, will be displayed on the browse screen of the CDJ-3000X.
If the upload isn't complete, the track is displayed but can't be loaded.
Upload the tracks you want to use using rekordbox for iOS/Android/Mac/Windows beforehand.
Uploading is not required for tracks from streaming services.

(3) CloudDirectPlay has been started with rekordbox for iOS/Android and tracks are uploaded to Google Drive.
For system reasons, even if you've started using CloudDirectPlay on rekordbox for iOS/Android, you must also log in to Google Drive on rekordbox for Mac/Windows if you've uploaded tracks to Google Drive. 
If you don't have a PC/Mac, please log in to Dropbox.

(4) Impact of maintenance or failure of Dropbox or Google Drive
When Dropbox or Google Drive is undergoing maintenance or experiencing problems, tracks can't be loaded. Try again when Dropbox or Google Drive is back online.