Playing in quick play, on "Charlene," with all four band members on super easy, and three of them signed into Xbox Live accounts, we got 100% on every instrument on our first try. "All Four One" (finish a song with four players) unlocked for all three signed-in players, but "Practice Makes Perfect" (full band 100%) only unlocked for the vocalist (band leader/file owner), and nobody else.
We tried several more times, got three more full band full combos, once with all members on medium, and still "Practice Makes Perfect" wouldn't unlock for the signed-in drummer or guitarist.
Over at xbox360achievements.org, you can see other people are having problems with this too. They don't seem to have any good solutions or theories as to a cause, either.