I was just about to post a comment about the exact same problem when I saw yours. On one particular song the Melodist and Soloist simply won't generate any notes even though theses features work fine on other songs and with the exact same styles and parameters. I don't know if I've inadvertently disabled something or what else the problem could be. I know that's not much of a contribution to solving the problem but at least you know it's not just you, lol.