I believe excess nitrogen in the soil is one of the most common causes so you may try adjusting your fertilizer. It can also be a temperature issue. Most tomato plants have a narrow optimal temperature range for bloom production so if the nights are still relatively cool where you are, that could cause a problem.

Have you tried gently thumping your blossoms to make sure they are being pollinated? That is the first thing I do every day when I check my tomatoes.