I think I'm reading you to say these are in the ground. It's common for clump growers to lose vitality at the center of the clump to the point where the center dies out entirely. Grasses do this as well. It's your signal that they need dividing. The reason is the root mass in the center of the clump is soo congested the plants growing there are unable to take up water or nutrients.
Dig up the clump. Chop it apart with a sharp spade, ax, adz ....., and replant. Use plants from the outer edges of the clump as they will be most vigorous.

Al