We can speak of the plight of Iraq's Sunni because they are all suffering. But there is no social, political, or military unification among Iraq's Sunni population. As the 2006 Awakening showed, a unified Sunni opposition could cripple ISIS. Unfortunately the Sunni are more fragmented than ever. [Carnegie-MEC]
The failure of the Shiite government to deliver security, the ineffectiveness of the Sunni parliamentarians to deliver on promises, the tribal divides (some do support ISIS) and a general feeling of hopelessness keep them from unifying. Fighting ISIS can be seen as a way to support an unfriendly Shiite government.
No popular leader has emerged and choosing where to put external support (money and arms) is problematic. It's been over a year since ISIS occupied Mosul. The stability that ISIS has delivered has even split Mosul into pro-, anti-, and indifferent groups. There seems no clear way forward for Iraq's Sunni.