I can't answer why you would have had less reseed this year, but I grew cosmos in my sun bed for years. After awhile they crossed and became inferior and I wanted to start again and I pulled all the plants at the end of the year and I haven't seen one this year. I would say if you want to continue to see them back every season, choose your best plants and collect seed and distribute the seed yourself after the first frost where you want them to come up. That gives you the chance to increase your reseed.