Probably it is too much rain.

With some apparently healthy tomato plants, you will have flowering but no fruit in periods of high humidity, which we definitely are having this year. The rain and humidity make the pollen 'sticky' so it clumps together and, consequently, pollination fails to occur. Some varieties are worse about having sticky pollen than others. In my experience here in southern Oklahoma, Brandywine is one of the worst for being slow to go from the flower to fruit stage.

To increase the odds of getting fruit, shake your plants gently. This is easy to do if you have them in cages or if they are trellised or staked. Just grab hold of the cage, stake or trellis and shake it a little. The shaking might loosen up enough pollen to cause pollination to occur. It is a little harder to shake them effectively without breaking a stem if the plants are sprawling on the ground, but you can still do it.

There are a couple of other ways to help increase pollination.

You can use your fingers to 'thump' each blossom several times. This is pretty easy to do if you only have a handful of blossoms per plant, but if you have tons of blossoms, it can become time consuming. On the other hand, you don't hve to thump them all.

Or, if you have your plants caged, staked or trellised.....and if you have an old tennis racket or badminton racket sitting around, just walk down the row of plants and give each cage, stake or trellis a couple of fairly hard whacks with the racket. That whack may shake the plant hard enough to induce pollination.

The only other option is that your plants have too much nitrogen so their energy is going into leaf production and not fruit production. Since you have flowers, though, I doubt that is the problem.

You should get great watermelon production IF the rain eventually stops. Whether the watermelons have good flavor will be the issue. Watermelons that ripen when it is extremely wet often have a noticeable lack of good flavor. Just like tomatoes, watermelon varieties have a certain brix (degee of sweetness) they generally attain. If excessive rainfall occurs, the flavor is essentially 'watered down' and you can't do anything to prevent it, except pray for the rain to stop before the watermelons hit the ripening stage.

Unfortunately we are at the mercy of the weather and this is just a really awful weather year. I hope the rain stops and the production of the plants improves.

Dawn