I am thinking it is the rock wall that is causing the problem. The wall traps heat and rapidly evaporates the moisture from sorrounding soil. So the peony next to the location of the rock is at a disadvantage. The color of the leaves of this peony is yellowish showing signs that the intake of nutrients is less. More frequent watering and adding more organic matter plus some slow release fertilizers is your solution.

Or move it back farther away from the wall, or both.

Thanks for the suggestions. I've heard Peonies aren't big fans of moving, so I am going to try the soil amendments, fertilizer and extra water first. Thank you!!

Well, here we are 1 year later. I tried the extra fertilizer, soil amendments, and extra water, but the Peony looks the same. Guess I'm moving it this fall or next spring!!