Hi there! The 9/12 Stars means you need to get 3 more Stars from the previous Island to unlock the next Island. Working on old Islands/Levels will not fulfill the Star requirement, you will have to work on the Island prior to the unlocked Island to get the Stars you need to unlock the locked Island.
Quick update for you all: Beandog got a little too excited and wasn't ready to share this update. This change has been temporarily reverted, but you may see it return at a later time. Sorry for the confusion!