Please make sure your music files have the correct file extensions (for example .flac, .mp3 etc.).
The lack of file extensions can especially be a problem if you have transferred your music from an Olive Music Server to a PC/Mac via XBMC. If the file extensions are missing we recommend to add the extension manually to a few files first, transfer them to the ONE and see if that solves the issue. If it does then you can use a variety of free software tools to add the file extension to all files automatically: