I'd say you weren't failing at all. If even the tail of the dragon is in the poison, they heal up. If the fight goes on to the extent where you run out of MP on any healer, but especially WHM, it's usually a case of the tank messing up, in the Brayflox dungeon. Also, f the tank doesn't sit right UNDER the belly of the dragon, a lot of times he's going to be hit with major poison stacks when the dragon uses its breath attack, so if the tank isn't doing this, it also could be an issue when it comes to healing.

So please, don't let this discourage you. Also, I go through Brayflox every so often on SCH, which doesn't can't cleanse at that level. I sometimes cast stoneskin on people to help combat the poison, but really, unless someone's stacks are really high, don't Esuna them, the MP cost isn't worth it, as others have said. Rely mostly on Cure 1 for this fight, Esuna when the poison stacks get to high, and you'll find you have less MP issues I suspect.