Paul, don't add any more acidics to your soil around them. In the fall top dress with composted steer manure (I get it in bags at big box store-- I think it has other barn sweepings in it too). Report back next year.
If the rhizomes get too congested, growing on top of each other then that is not a good thing and they decline. I have had this happen many times before I even see a bloom. I pretty much have it figured out now.
Another option is to try a large container and a good potting mix thus avoiding the native soil.
