Is the ground waterlogged?
Some things that affect phlox are spider mites, powdery mildew. Have you checked for those? A picture might help. Otherwise, you may get more response on The Garden Clinic forum.

Here is a link that might be useful: The Garden Clinic

I think they might be too wet -- they like to be a littler dryer. I made the mistake of planting one in a lower area of my wetish yard, after being moved, it flourished.