There is a form of the grey mould fungus Botrytis that attacks peony buds and causes them to stay closed. They soon become covered with the typical grey spores. Peony buds can also fail to reach full flowering stage because of drought, frost damage, too deep planting or root disturbance. Take care when weeding or digging around plants, keep them well watered in any spring droughts and, if possible, cover with horticultural fleece if frosts threaten. In severe cases the entire plant can wilt with little prospect of recovery.
Photo credit: RHS