When purchasing Tomato blossom set, ensure that you buy blossom set, and not just the "blossom spray"...check the ingredients. Beware of the brands that do not contain the hormone that causes the flower to produce when conditions are not favorable. There is a popular brand out there that is sold in the big box stores, and elsewhere, that simply contains some very small nutrient content at a very large price....about 6 bucks for the equivalent of about a teaspoon of fertilizer. It contains ONLY a bit of fertilizer, has no hormone, and probably will do little or nothing to solve your problem.

Darwin