September 2014
After completing a song, I click on the mailbox to license it, but the "License Song" button is greyed out and a pop-up message says "Bella has already licensed a piaon song." She was able to license only one song and is at the max level. I have the same problem with my sim who has maxed out the violin.
September 2014
I don't know any workarounds but if it's indeed a bug you can always report it here:
http://forums.thesims.com/en_US/categories/possible-bugs