Put a trowel into the soil by the wilting one. I'll bet it's dry 4 inches down.
Linda C

I have had trouble with echinacea wilting...eventually turning brown and dying. See my post on Pennsylvania gardening...I just did a google this morning on diseases of echinacea and found a site that describes what I think is the problem with mine. There is a link in that post. Scroll all the way down to today's message. Don't know if it applies to your situation but it was the first sign I've had of what looks like my wilting problem.

Here is a link that might be useful: Link to PA post

I have three in a row, and the middle one always wilts and needs more water than the other two. I've never figured out why - maybe there's something compacted under it.