Answer: Since I don't know how you have been caring for them up until now, I can't tell you exactly what is wrong. Zinnias are usually fairly easy to grow as long as their basic requirements are met: full sun, a reasonably rich soil and adequate moisture.
There are several possibilities I can think of right away: one is that they are still too young to bloom, another is that they are not receiving enough sunlight, and the third is that they have been provided with too much nitrogen. (Over feeding will result in lots of lush leaves and very few flowers.) Since your plants seem to be growing well it doesn't sound like watering is the problem. If you suspect over fertilizing is the problem, be sure not to feed them any more.
The following tips may be helpful for next year. Basic soil preparation for annuals such as zinnias includes working in a generous amount of organic matter along with 2 or 3 cups of a balanced granular fertilizer (such as 10-10-10) per 100 square feet before planting. Any additional amendments would be determined by a soil test. (Your County Extension can help you with the soil tests and interpreting the results. Their telephone number in NYC is 566-0673.) Some gardeners may give their annuals a light mid-summer side dressing of fertilizer or an occasional weak foliar feed, but as a rule heavy fertilizing is not required, especially if the soil preparation is done correctly.
Good luck with your zinnias!
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