some folders are unseen or unknown

i have a problem with my sansa express 2g.
I’ve loaded some music and when i want to play an artist, it’s in the unknown folder.
not all my music have this problem, but 2-3 albums (at the moment) are unknown which make it hard to listen to.
why is that?
I’ve checked the id3 tags and all fileds are filled.
so how does sansa sort this folders?
a second problem i sthat there are albums that doesn’t even get to be unknown. they are just not there.

Usually this is due to a bad ID3 tag.  Best thing to do in most cases is to delete the Artist field from the files and then add it back in.

For files that are just not showing up, make sure that they can be played on your computer and that they’re in the WMA or MP3 format.  You can also try rewriting the ID3 tag using the same method as described above.