Band hero does not give the achievement "Treasure Hunt", to achieve this it was necessary to collect the other 19, I did it, but the achievement of those 19 I have not received. Platform Xbox 360.
I had this problem when I first tried to get the Platinum on PS3. It's a glitch, and a friend of mine told me a great trick. You need to play Vocals on Beginner, and try to 100% every song from the list of the secret notes. Once you've played through every song that it asks, you'll receive the achievement.
I hope this helped. c: