I suspect you haven't done one of the strings of optional quests involving the elementals (mossback, etc). Mossback himself has four places where you need to do quests for him to open them all up. You need to help him everywhere he appears, but the other elementals too.
If in doubt, go through the areas in order that you were able to see them and if there is an elemental, try to help them. If you can't talk to them and you didn't help them already, you probably missed one earlier.