I completely agree with Zoe. How do the leaves look? If the leaves look like the petals, the only chance you have are at the leaf nodes. I would start again and clip the flowers off, make sure the roots aren't over grown and packed full. If you get these flats too late, it is hard to get a good plant out of it. Since it was just planted, you could easily take one out and examine the roots. You could also get a pack of zinnia seeds and direct sow them.
