Cosmos don't need fertilizer - if you're feeding you're likely causing the plant to expend all its energy producing foliage.

Cosmos thrive in full sun and will flower more heavily in poor soil than in rich. Require little water and little attention.

Stop the fertilizer and you will probably get a late crop of blooms.

Cosmos are day-length sensitive, too. Many varieties flower late in the summer as the days get shorter.

Daylength sensitive means that the plants need a certain amount of daylight to stimulate flowering. So those varieties of Cosmos will grow but not flower until the daylength initiates flowering in the plant. Cosmos need SHORTER daylengths to start flowering.

I know the older variety called Sensation Mix is daylength sensitive.

Some of the newer varieties of Cosmos are bred to flower without regard to daylength. Check the seed catalogs/websites to find out those varieties. Try Sonata, Versailles, or Gazebo Cosmos varieties.

Chris

Last year I had one grow 8 feet tall and never bloomed.

I attempted to keep the root, overwintered in a pot, but I couldn't get it to grow.