Sometimes the system can be finicky about refreshing the song database, like if you just add one or two songs to the USB folder the system may not see them.

I might be doing something wrong, but I almost always have to delete the entire USB stick, plug it into the bike and have it show "NO SONGS FOUND", and then go back and re-copy all the songs to the USB. Kinda a PITA but luckily you don't have to do it often and it works flawlessly once it's all setup.