The massacre of hundreds of members of the Albu Nimr tribe by ISIS was seen as potentially driving other Sunni tribes off the fence and into the arms of the Shi’ite government as an alternative to ISIS. It’s not working out that way.
Albu Nimr, a major Sunni tribe in the easternmost portion of the Anbar Province, was also one of the few that was still on good terms with the Iraqi government, though their fighting forces have been effectively wiped out in the ISIS purges around Hit.
They were trying to bring other Sunni tribes on board for expelling ISIS from Anbar, but there’s been no sign of any of these other tribes moving closer to the Iraqi government since the massacres, and they may well have had their intended effect of convincing the other tribes not to get involved.
ISIS has mostly relied on local Sunni tribes to run its expansive territory across Iraq and Syria, though on those occasions they have clashed with tribes, they’ve been extremely heavy-handed, trying to make an example out of the groups that resist their rule openly. It’s a dangerous game for ISIS to play as well, because they probably can’t fight too many tribes at once, and their economy is heavily dependent on keeping them at least partially on board with their continued rule.