Flowers sometimes wither and break off at the knuckle. Pollination of the flower has not taken place. The cause is usually dryness at the roots and in the air.
Ensure that the soil or compost is not allowed to dry out by regular watering. Spray the plants in the morning to increase humidity. Gently tap the tomato plants when in flower to aid pollination.
There is no product on the market to prevent blossom drop.