If you applied it in the prop wash, that's easily the best method, so I would think it has to be your pH. Sometimes it may take a couple applications to get a good bloom, but since you applied in the middle of the summer, I would have thought you would have gotten a bloom.
I would definitely check the pH first, and if it's low, add some lime (not hydrated, which will kill the fish). Then once the pH elevates, you can fertilize again. If the pond is only 15% covered with FA, a successful fertilizer application will kill the FA within a couple days as the plankton bloom cuts out the sunlight penetrating through the water.
Your bluegill are good-sized for the pond not having been fed or fertilized until recently. You should be able to grow some real monsters once you get a good plankton bloom and they figure out the pellet thing.
