Q.Drooping coneflowers
I have 3 coneflowers planted 5, 3 and 1 year ago. They have buds and should bloom soon. In the morning they’re all are upright. The last 2 afternoons, about half the stems on the oldest one have drooped – like an upside down U. Today, the middle one is also starting to droop. I’ve looked under them but there are no signs of animal damage or any sucking type of insect. What do you think is the problem?

The Coneflowers droop as a mechanism to conserve moisture by presenting less surface to the sun.
They are a prairie flower that like to be on the dry side. They will show sun wilt then recover in the evenings.
https://www.gardeningknowhow.com/ornamental/flowers/coneflower/growing-purple-coneflowers.htm