Coneflowers like well-drained soil. I suggest replanting them at the top of the slope. Make sure to do early morning or late afternoon, so they don't get sun shocked. I believe they probably are getting too much water, hints the bottom of a slope and rotted looking roots. That doesn't mean they don't like water, ha, they just don't like to bathe in it. Look for wild flowers in your area to plant near a pond at bottom of your slope, they love areas like that because it's their natural environment.