I have similar issues. I moved into a house with three peonies three years ago. The largest one blooms every year, although fewer buds this year. The middle-sized one comes in more slowly, so I think it is a different variety – but it has not bloomed in the three years. The smallest one was accidentally dug up the first year, and I suspect the disturbance hindered it blooming – this 3rd year it has one bud.
Maybe the middle one is buried too deeply – how can I tell?
Every year, they all succumb to powdery mildew mid-summer, when the heaviest humidity starts up – most peonies in my neighborhood do. The biggest one gets it first, then the middle, then the smallest. I cut back the foliage, dispose of the waste thoroughly, and spray with baking soda water. I heard that helps with fungus. Then I fill the "holes" in the border by putting potted annuals in front of the peony spaces (not on top). They are all in full sun.
The plants are at a similar stage of development as others in my Zone 6b area of southeastern PA, and the buds I have should open in the next week. But why so few blooms?