Answer: Tomatoes stop setting fruit in the summer heat. It is not just the daytime temperatures, but the warm nights as well that causes the flowers to abort.
There is little you can do except wait for fall. Some Texas gardeners will start a new plant from the old ones this time of the year. Take the end of a vine and lay it in a hole with about 6" of the end sticking out. Cover it with soil and water the spot well every day or so for a week. In a couple of weeks, you can cut the new plant from the "mother plant" and remove the mother plant.
By September your new plant will be producing for you. Make sure and keep mites and other pests under control. Keep the plant growing vigorously with occasional fertilizer applications.
Good luck with your tomatoes!
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