Rekordbox doesn't seem to export certain files correctly

My music library is organized into folders corresponding to a producer's name. Within each folder, there are then albums by that producer. I make sure to have the "artist" and "album" fields populated correctly before right clicking on the artist name in the tree view and export the files as a playlist. Then I get a playlist per producer on my USB stick.

For the most part this works correctly. However there are certain tracks in my library that do not show up in the playlist for the artist. When I look at the files on the USB stick in the "Contents" folder, it seems Rekordbox puts a lot of these in an "Unknown Artist" folder. They are wav files and I'm not sure why certain files consistently go into this folder and not under the the folder named after the producer's name. Do you have any idea why this might be? The tracks seem to have exactly the same artist name and album name as the others in the folder that get sorted correctly.