My guess, and that is all it is, it is too deep.
I planted transplanted some peonies and while not in such a short time span, after flowering comparatively well for several years, they now look a lot like yours.
It takes years for a peony to really take hold so I raised mine up, which while the guides say put them in at this level, it is better to be too high than too low.
I have cleared mulch and dirt from some that did not seem to be doing as well as they should but then they were sitting in hole compared to surround area and it does not take long for that hole to fill in.
I am moving /raising a quarter dozen this fall, a good deal higher to see how they perform next spring.
Mulching peonies I found is not a good idea, yet I have some on the edge of my vegetable garden and even though they seem to be covered with enough dirt to hurt them they are actually getting to big and spreading.
Another oddity; when my parents bought their house the yard was full, dozens of peony bushes, the yard line on one side was determined by a peony hedge, and they were in a drainage low spot.
Well they ripped out all but a dozen.
NOw where lillies of the valley have been for as long as I have been alive, peony bushes are popping out of the ground all over the place and actually becoming a weed.
I was not born till years after they bought the house but if dad was still alive he could tell me where they used to be so I could know where more may pop out.
As a guess again some types must be more sensitive to location, as some planted in an identical manner and location do poorly to middling while others do well.
