If your soil is heavy clay, and you give your plants too much water, the soil stays too wet for too long a time, and the roots rot away. A plant cannot live without roots, so it dies.
Soil can vary from one place in a yard to another. Also, if one plant is in a part of the yard that's higher than the other, the water will drain away faster, which could explain why one plant is doing well while the other is failing. It's very difficult for someone to say, when he/she isn't actually at the property. I would suggest that you post a photo, except in your case, the problem might not be discernible just from a photo.
The symptoms of overwatering are the same as the symptoms of underwatering, i.e., yellowing and dropping of leaves. In your case, since you mentioned that the soil wasn't drying, overwatering could well be the cause of the problem.
IMO, treatment with a fungicide will only help if applied at the time of planting, and it won't ever do any good for root rot. Symptoms don't usually appear until the roots are almost completely gone, and by then, it's almost always too late to save the plant. It isn't unusual for the plants to be infected before you buy them, so it's important to examine your newly obtained plants very carefully, before putting them in the ground.
You can dig up the failing plant and examine it to see if the roots look healthy. You can examine the stems where they sprout from the crown, to see if they are wet and mushy. (Just as an aside, crown rot often results from planting the plant too deeply in the ground. You can easily check for that, too. IMO, there is no cure for it.)
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams