Yes that is correct. Cosmos will not flower in over fertile soil. My feeling is that your nitrate levels are too high. It may be a good idea to try one of the following:
1. The easiest option is to take them out and plant something else.
2. Test the soil for pH (acidity/alkalinity), N (nitrogen), P (phosphorus), and K (potassium). This can be done with a soil test kit which you can purchase from your nursery or hardware store. This will indicate which soil elements need correction.
3. Mix a quantity of washed river sand thru the soil in equal parts. 1 soil to 1 sand. This will change the existing structure of the soil and lower the available nutrients.