i think i have the same problem my tomato buds flower then the flower turns brown then the stem turns yellow and falls off , and there is not a small tomato on the bud . is this the same problem you are having

High temps. Temps above 90 F for periods of 2 days or longer will do this. The plant is just trying to survive. Fruiting take a lot of energy and water so the plant will drop flowers so the plant can live to fruit later when the temps are consistently below 90 F.