There are many reasons peonies fail to bloom other than "They were planted too deep". Many questions should be asked before a logical conclusion can be drawn as to why a plant has failed to bloom.
How long has the plant been in the site?
In the image above, notice the abundant flower buds on the three year old plant compared to the developing one year old plant. Be patient, let peonies develop their roots systems over time before expecting blooms. Young plants may not have an adequate root system with sufficient food reserves to produce a bloom.
How Big Was the Root that Was Planted?
Imported roots are much smaller than most domestically grown plants. Smaller roots may require a few more years of growth to produce an adequate root system with sufficient food reserves to produce a bloom. A good peony root should have a minimum of three to five 'eyes' (dormant buds) for growth and at least three corresponding roots measuring four to six inches in length and one inch in thickness.
Is the Peony Bush Growing in Full Sun?
Many times a peony was planted at the same time as the rest of the landscaped yard and is now being shaded by trees that have grown much larger. Peonies need a minimum of eight hours of sunlight to bloom successfully.
Was it a Wet Spring?
Tender new growth with developing flower buds can be attacked by a fungus in the spring during long periods of rain and high humidity. Signs of this disease appear on immature flower buds about the size of a pea; flower buds appear brown or blackened on tips of new growth and fail to develop. First and second year plantings are most susceptible to this disease. Cutting down and removing peony foliage in the fall is still the best preventative measure for disease control.
How Deep Did You Plant the Peony Root?
Peonies produce dormant buds (called 'eyes') at the base of each stem in August. These dormant 'eyes' produce a peony bush the following spring and should be planted 1 1/2" to 2" below the soil surface. If in doubt about the planting depth of an established bush, carefully scrape the soil around a few stems in the fall to check the depth of the young 'eyes' at the base of each stem. It is rare for a peony bush to be planted too deep and be the reason for lack of blooms; in our fields we are planting thousands of roots each fall without measuring planting depth as we drop them into a furrow. Roots have a tendency to self-correct their planting depth by producing dormant eyes at the proper depth as they mature. Too often quick advice is given and peonies are dug up and replanted without knowing that disturbed peonies are often set back, with blooms delayed again and again. Peonies do best if undisturbed once planted.