Dianthus require very good drainage and will not tolerate soggy soil or "wet feet" :-)) I would assume yours are declining due to excessively wet soil....at least for a period of time. If that is the case, there is really nothing you can do for them. When/if you replant, make sure the soil in the area is very freely draining, even during the rainiest periods. Or a raised bed may make sense.

Many varieties do tend to be rather short-lived and dividing every 3 years or so will help to revigorate aging plants.

Thank you for the comments. I thought is was strange that all of the plants were not dying together as they're all planted next to each other. Since i created the post I've lost another 1.5 plants. I'm thinking about taking the advice and replacing with something a little hardier in the fall. Any suggestions for something that will spread similar to dianthus and stay ~6-8" tall? Maybe creeping phlox?

Dave you might want to ask for suggestions on the perennial forum rather than the shrubs.

ok, sorry about that

No need to apologize - It wasn't a criticism, but you might get more suggestions.

I can't suggest reasonable alternatives for you since my growing area is so different.

I've lost over 8 plants and it sounds like it's a drainage problem which I read above. I guess it's time to put something else in its spot.