Even the best of gardeners fall short of finding that perfect tomato fruit. Failure to set fruit can be caused by many different problems.
The night temperature might not be within the correct range, 55-70 degrees. When the day temperature exceeds 90 degrees, fruit set tends to be at its lowest. Dry soil can cause blossoms to dry up and drop. Too much nitrogen can lead to more leafy growth at the expense of fruit and flowers.