There are three possible causes as follows:

(1) Cloud Sync is incomplete
If the Cloud Library Sync between rekordbox for iOS/Android/Mac/Windows and the library in the cloud hasn't finished, the track won't be displayed on the browse screen. Wait for the sync to finish, then the track should display.

(2) The track hasn't been uploaded to Dropbox
Only tracks that you've uploaded to Dropbox will be displayed on the browse screen.
If the upload isn't complete, the track can't be loaded.
Upload the tracks you want to use to Dropbox beforehand, then use them once the upload is complete.
You can upload with rekordbox for iOS/Android/Mac/Windows.

(3) Impact of Dropbox maintenance or failure
When Dropbox is undergoing maintenance or experiencing problems, tracks can't be loaded. Try again when Dropbox is back online.
You can check the status of these issues on the Dropbox website.
For details, see [dropbox.com].