Mutual distrust blocked arming Sunni forces against ISIS.

Evidence

Many who were alienated and resentful of the Shiite-led central government sympathized with the Sunni militants. Because of mutual distrust, the central government was reluctant to arm trained Sunni security forces for fear that they would sell their weapons to ISIS or join it themselves (Hubbard, 2014). — Albert Bandura, Moral Disengagement, 2489