No blossoms on cosmos plants that look very healthy otherwise. These plants are self seeded in raised garden plot.
No Blossoms on Cosmos
Certified GKH Gardening Expert
Cosmos plants take a while before blooming, so depending on when you started them, they simply may not have reached maturity yet. Too much nitrogen in the fertilizer is also a common reason.
This article may help you determine the reason your Cosmos plants aren't blooming: