# Does Thunder Buffer only work once per round?

My friends and I recently had an issue with the sorceror ability Thunder Buffer.

Daily, Arcane

Minor Action, Personal

Effect: Until the end of the encounter, you gain a +2 power bonus to AC, and as an immediate reaction you can push each enemy that hits you with a melee attack a number of squares equal to your Charisma modifier.

I thought the ability worked as an immediate reaction once per round on an enemy that hits the sorceror with a melee attack. However, the sorceror pointed out that the ability says "each enemy that hits you". He argues that he should be able to knock back every enemy that hits him with a melee attack in a given round.

So my question is, which is it? Or does the ability work some other way entirely?

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The problem is that the action is described as an immediate reaction. immediate actions are defined as once/round effects: