looks like they need more water...touch the soil with your finger if you are not sure..if it is too dry it needs more water...
also..if the leaves are turning yellow, it means over-watering.
if they start to turn brown(dry)at the tips, they need more water.

zinnias need plenty of sunlight, heat and regular watering..I live in NJ, I plant them every year and they bless me with a blast of blooms.. :) good luck!

check the drainage too...if excess water is drained properly...

Thank you!!!! :-)

Is this plant on a terrace? I see a lot of brick and concrete. Keep in.mind, official temps are recorded off the ground, in the shade--around here, those temps have been quite hot (upper 90's), they can get much hotter on a terrace without a lot of other plants or evaporation. Plus, you have to deal with wind. I know because I used to have to garden on a terrace. It's a challenge!