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Potential Causes of Blossom Drop
The primary causes of blossom drop in tomatoes are environmental (e.g., temperature and relative humidity [RH]) or cultural (e.g., lack or excess of nitrogen [N] fertility). Secondary causes can include lack of water, reduced or extended light exposure, excessive wind, insect damage, foliar disease, excessive pruning, or heavy fruit set.
Primary causes of blossom drop

Temperature: Tomato plants drop their flowers under extreme temperature regimes, such as high daytime temperatures (above 85°F), high nighttime temperatures (above 70°F), or low nighttime temperatures (below 55°F) (Table 1). Optimal growing conditions for tomatoes are daytime temperatures between 70°F and 85°F. While tomato plants can tolerate more extreme temperatures for short periods, several days or nights with temperatures outside the optimal range will cause the plant to abort flowers and fruit and focus on survival (Mills 1988). Temperatures over 104°F for only 4 hours can cause the flowers to abort. If nighttime temperatures fall below 55°F or rise above 70°F, or if daytime temperatures rise above 85°F, the pollen becomes tacky and nonviable, preventing pollination from occurring and causing the blossom to dry and drop (Chester 2004; Levy, Rabinowitch, and Kedar 1978; Mills 1988; Ozores-Hampton and McAvoy 2010).

Low temperature: Low temperatures interfere with the growth of pollen tubes, preventing normal fertilization. The pollen may even become sterile, which causes blossoms to drop.Tomato fruit do not set until nighttime temperatures are above 55°F for at least two consecutive nights (Chester 2004; Ozores-Hampton and McAvoy 2010).

High temperature: Sustained high temperatures, especially at night, rapidly deplete the food reserves that are produced in the tomato during the day. The result is sticky pollen, altered viability, and poor or no pollination. Ultimately, the blossom dries and falls off. Female flower parts can also undergo morphological changes, such as drying of the stigma (Mills 1988; Ozores-Hampton and McAvoy 2010).

Relative humidity: The ideal RH for tomato growth and development ranges between 40% and 70%. Relative humidity plays a major role in pollen transfer. If RH is lower than the optimal range, it interferes with pollen release because the pollen is dry and unable to stick to the stigma. If RH is higher than the optimal range, the pollen will not shed properly (Mills 1988; Ozores-Hampton and McAvoy 2010).

Nitrogen: High or low application rates of N fertilizer can cause blossom drop. High rates of N encourage the plant to produce excessive vegetation at the expense of fruit set. Low N produces spindly vines with low food reserves that cannot support a tomato crop (Chester 2004; Levy, Rabinowitch, and Kedar 1978; Mills 1988; Ozores-Hampton and McAvoy 2010).

Secondary potential causes of blossom drop

Low or high soil moisture: Tomatoes have deep roots that can penetrate up to 5 feet. Low soil moisture stresses and weakens the plants. The root zone should be kept uniformly moist throughout the growing season to develop a large root system and reduce plant stress (Chester 2004; Ozores-Hampton and McAvoy 2010).

Heavy fruit set: When a tomato plant has produced a large number of blossoms, the resulting fruits compete for the limited food supplied by the plant. The plant will automatically abort some flowers. Once the initial crop is harvested, the problem should subside as the plant's nutritional status comes back into balance (Levy, Rabinowitch, and Kedar 1978; Mills 1988; Ozores-Hampton and McAvoy 2010).

Wind/pruning: Excessive wind can desiccate flowers and/or physically knock them off, reducing fruit set. Excessive pruning can reduce the amount of energy the plant produces and thus can reduce flower production and fruit set.

Light: Lack of sufficient light or extended exposure to light can reduce fruit set.

Insect damage or disease: Growers should use adequate cultural practices to control insects and diseases. Fungal diseases—such as botrytis, heavy bacterial spot, or bacterial speck pressure—have a negative effect on fruit set.

P.S.
What size reservoir/water volume are you using? How much water are the plants drinking daily? Are you replacing what they drink with fresh water daily?