Apparently Big Boi and Andre 3000 were supposed to appear on Frank Ocean's "Pink Matter" together. However, Andre prevented that from happening because he didn't want an Outkast reunion to be done on another artists album, according to what Big Boi said in his tweets. He also said the same thing happened on a song that Andre did with T.I. as well.
Check out Big Boi's tweets here addressing his absence on Frank Ocean's "Pink Matter".