It happens when there's a metric crap-ton of stuff going on. I think that it's because the game is too busy registering everyone else's attacks at the exact same time that it doesn't register yours. It happens to me almost every Odin or Behemoth FATE--I'll nail them with Fire-X and Blizzard-X but never see my Astral Fire or Umbral Ice states kick in. It's frustrating to say the least, especially when I finally manage to see one of them, only to have them disappear again.