I’m guessing the ID3 tags. Search the forum for all kinds of information.
I agree. If the ID3 tags are missing, wacky with foreign characters, Comments field full of Lord-knows-what, or of a version other than ID3 v2.3 ISO-8859-1, then your Fuze can’t ‘see’ it. It files all these under “Unknown” Artist in the lists you see on your player.
If your firmware is up to date, Folder browsing is available and your missing music should be able to be found and/or played from here. But it’s also a good idea to fix your tags.