Some mages cannot acquire Frozen Orb. The mob despawns sometimes and there is no recourse for a player to summon it again.
Please, Blizzard, it really sucks to kick off phase 4 and not be able to get our runes.
See Some Mages cannot get Frozen Orb Rune: SERIOUS BUG - #7 by Daejopa-crusader-strike for more information.