For what it's worth, Watcher said on Twitter yesterday that the Dark Shaman set has a shared drop lockout across Normal/Heroic and Flex where you have only one chance a week. Whoever said that you get multiple chances was mistaken, so no more need to do flex pugs for people that clear normal/heroic every week.
I have killed the boss every week since 5.4 went live and still haven't seen it.