Even if you interupt perfectly, one eruption around 66% will go through, because at the beginning of the fight, the only spell ifrit casts is Eruption, so he eventually fully resists it. Once he starts casting plumes, you should be able to interrupt every eruption. If you can't get every one, then either one of your dps is using a stun (mnks and drgs have one they can use in rotation that is a dps increase), or you are just lagging/bad, because as of yet, you can't miss stuns.