As prairie dropseed is hardy to zone 3, it seems unlikely it suffered winter damage. The plant is also seldom bothered by pests. Yet, you might dig up the saddest of the problem plants and check its roots for nematode damage. Bumps on the roots and often a lack of smaller feeder roots could be a sign of this nearly microscopic pest.

There are lots of different species of nematodes, a few of them are beneficial, but this SITE may help you deal with them.

At five years I would call it old age of a perennial. Time for division or fresh plants.