Answer: Echinacea is relatively easy to grow in well draining soil in a sunny site. While they will tolerate some shade, they won't flowers as well as they could in a sunnier site. There are no particular disease problems that would cause what you describe so I suspect it is environmental. Late season cold damage, perhaps? In any event, I would cut off the flowers that look puny to force the plant to produce healthy new flowers.
Best wishes with your purple coneflowers!
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