My hydroponic tomatoes were dropping blossoms. Lack of pollination is one possible cause. Most of what I read online said tomatoes self-pollinate quite easily. Wind pollinates them naturally outdoors. I had setup a fan to blow on them briskly every 10 minutes. I figured that was plenty.
I did a test using a sonic toothbrush to vibrate the blossoms on one half of my plants. (In nature, the vibration of insects wings releases the pollen.) A few days later, those plants started setting fruit, while blossoms continued to drop from the other plants. So it appears in my case at least, wind was not enough.
I had two very different varieties of tomatoes growing, both had the same symptom and both responded the same way to the manual pollination.
My conclusion, self-pollination is not assured, and even wind may not be enough.