“Blossom drop” can sometimes be related to the weather — temperature (high or low — usually too low), humidity (high or low — usually too low), or water (high or low — usually too low). Not much you can do about the weather, I’m afraid, but if the soil is getting dry, try watering it every other day (except when it rains).
Nitrogen levels are a definite possibility. Are the leaves turning pale without becoming splotchy? That’s a classic symptom of nitrogen deficiency. If you don’t have that, it could be nitrogen excess, which causes the plant to focus more on leafy growth and less on its flowers. Both can cause blossom drop. The way to make sure with nitrogen issues is to send a soil sample to your local ag office for analysis; it’s cheap and well worth it.
Since your flowers aren’t opening, it’s not due to a lack of pollination, another potential cause. So we can rule that out.
One additional option worth mentioning, although I think you would have brought it up if you had noticed it, is insect damage. If a tomato plant is too stressed by insects, it will drop flowers to save its energy. Give your plants a good inspection for bugs and see if you see any.
Thanks for your suggestions...
We live in the Bay Area so the weather (rain) is not an issue - it never rains here in the summer. I use a drip system to water and I water everyday - could it be too much water?
Leaves look fine.