Sometimes you get a skill combination that has no off-hand essences, which results in no off-hand items dropping at all. It’s still possible to finish the event this way, but it unnecessarily extends the play time due to the missing damage from that slot.
Either the combination of skills should be chosen in a way that guarantees that every gear slot is covered, or the slots with no essences should still drop but with different bonuses that aren’t tied to specific skills.