Your plants do look like they are struggling.  First, none of them are shade plants, they all need at least 6 hours of sun.  Second, these all prefer well drained soil.  A rain garden usually doesn't have well drained soil. 

If you want to keep these plants, move them to a different area of your yard, preferably full sun and well drained soil.

Another thing to consider is you soil for this rain garden, using a trowel or a shovel, dig about 4 inches and down and grab a handful of soil.  Squeeze the soil and see how it feels.  Since this is a rain garden, I would expect to feel wet and for the soil ball to stay together.  Do you know what kind of soil you have - clay. loam, or sand? Knowing this will help you choose the correct plants.  Your plants may look like they do because they are too wet.  Exploring your soil is going to give you a lot of information.  

We have a very helpful website when it comes to choosing plants: https://www.canr.msu.edu/nativeplants/

And the Washtenaw County Master Rain Gardener website has a lot of helpful information that is free:  https://www.washtenaw.org/675/Master-Rain-Gardener-Class

Thank you for your question.