Rarely annihilation is not casting, it just does nothing and goes to cooldown, Or skill does nothing, icon stay enabled, and pressing the skill key just trigger nothing but behind the scene it's actually on cooldown. I do use lower cooldown, larger area, lightning and dual casting runes. also related to that last rune the dual casting doesn't always trigger. Sometime i cast the first i see my willpower drain and the area shows up and then cast the second one on another group and it does not display the area but it drain my willpower. Also the second cast is not always possible even if you have enough willpower and there is no warning / note in the rune saying what is the opportunity window to cast the second one. I have by habit noticed that when the cooldown reached 4 seconds left you wont be able to cast the second annihilation anymore.