Answer: Assuming there are no changes and you do not live in an extreme climate, there are a couple of things that might cause them to stop blooming. First, they might be too crowded or they have exhausted a poor soil. In either case here's what I suggest:
Immediately sprinkle a handful of 12-12-12 around each plant, scratch it into the soil and water well. Do this again in August.
In fall dig up the entire clump, divide it and replant. On an old plant you might get from 6 to 12 good divisions. Each division should have no more than 5 or 6 'eyes' (pink buds) visible. After it is dug and divided, fortify the old site- add a cup of bone meal and a good handful of 12-12-12. Mix in some compost, some manure, some sand etc depending on your soil condition. Mix well and replant at the proper depth. Next year at least some of these divisions should bloom, but if not, it make take another year.
Good luck with your peonies!
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