why is one playlist works but other playlist is empty

SanDisk MP3 players SanDisk® Clip Sport
12

I have the same problem. Or worse, I don’t get any playlist to work. I’ve created them with Mp3tag, VLC, manually… I even let Mp3tag create one from the files on the player. No go. When I select the m3u file I get “File format not supported”. Neither does it show up in the Playlists folder as I read it should.