Certified GKH Gardening Expert
Are they actually not producing seed or you aren't seeing new plants in the spring? If you aren't getting new plants it could be they dropped seed, but the birds or other animals got them all. If you aren't seeing any seeds being produced, it could be due to a shorter growing season because of environmental factors. Also, some birds, and even squirrels, will eat immature seeds before they are ripe.
Make sure growing conditions are optimal so the plants will go to seed. If you are deadheading, make sure you leave some of the plants undisturbed. You also can supplement with new seed if the reseeding wanes.