Have you been regularly adding compost or manure to maintain soil fertility? Is the peony still in full sun or has a tree grown enough to shade it part or most of the day? Has the soil built up over the roots making it too deep now? Twenty years is not too long without dividing, but you can divide in the dormant period, which will give you a chance to improve the soil. Al

Peony is still in the same place gets sun from about noon. The root is still in about the same depth of soil it always has been. I will try putting some manure in and around the plant and divide in the fall this year.

Thank you

Perhaps the top dressing has gradually made the tubers too deep. They don't like to be far below the soil.