Did you maybe edit some songs on the new device before it was fully synced?
In any case, the best way forward is to restore a full backup file on one of the devices. This will sync back everything into the other devices automatically.
If you don't have a backup file and are running the app on a Mac. You could try to restore a Timemachine backup of the ~/Library/Containers/Stage Traxx 3 folder.
If you have neither of those, you are out of luck. Don't forget to create regular backups.