Experts and BHG readers answer.
tomato blossoms do not set fruit
Tomato plants can drop their blossoms for any of the following reasons: too high or too low temperatures, lack of pollinating insects, too much or too little nitrogen, too much or too little humidity, lack of water, or stress from insect or disease damage.
In your case, I would bet it's related to high or too low temperatures or a lack of pollinating insects. I'd try an artist's brush to see if you can pollinate the blossoms yourself by brushing pollen on the flowers. If that doesn't help I bet it's related to your local weather conditions.