Have they been moved recently, if so it can affect flowering, are they getting enough sun, have you watered them and have you planted them too deep, they need to be planted just below the surface, all these could be factors in them not flowering, julien.

Also what age are they? Peonies do not flower until they are around years old.

I agree with Julian re the planting depth. If too deep, they will produce just leaves. I raised a few for a customer who had the same problem. I re planted just below the surface and it cured the problem. But I also agree with Moon G. How long have you had them? They do take a few years to mature enough to flower.

Oops should have said 5 years old!