Where are you, exactly? I don't recall there being any anti-dragon shields at sentinel island (though you are blocked from flying / swimming to the fjords, etc).
Did you fly around the various levels of platforms and make sure there are no ballistas left? Presumably if there were any nests left you would start seeing opponents on your mini-map, at least.
Try transforming into human form above one of the platforms, over a couple trees with nests containing treasure chests or the tree with a ship in it (where you will find a quest item, that you might want to use yourself instead). Maybe transforming back into a dragon will trigger something.
Did you try using the teleporter shrine outside the battle tower? The one that was beside the locked door when you were trying to get into the tower is above the poison gas.
