It's not uncommon for tomatoes to drop their early flowers. Temperature and humidity early in the season can fluctuate causing flowers to drop. Also, excessive nitrogen fertilizer is another possible reason. While it (N) will get the plants to grow initially, they like a bit of extra potassium (K) to set fruit. So the type of fert' you're using is possibly a contributing factor. What are you feeding them with and have the temps/humidity been excessively low or high?

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The secret is the soil.