i dont know why it would be happening, but theres somethings I can suggest you try. (to try to fix it)
run check disk on the device, or back up the songs and format it (this deletes everything at once) and reload your songs. Are they mp3 or wma? Wma files save a bit more space at the cost of “quality” of the music, not many people can tell the difference(unless your really into music/audio).