Hi,
BubbleUPnP only list folders and tracks as your Syno NAS media server enumerates them and return them to the app.
If files are missing:
- the media server could not scan them for some reason (unsupported format, file/folder permission problem, ..>)
- the file tags are incomplete (though they should be visible browsing by folder)
- you need to do a manual rescan (via the Web DSM interface, in top-right menu > Media Server)