Assuming that the plants are getting at least six hours of sun a day, the next blossoms you get, tickle them so that they'll self-polinate.

It is called "blossom-drop". Search that term and you will find all kinds of discussions about it. There is a FAQ here to that covers it, its causes, and things you can do to help well.

Dave

Here is a link that might be useful: Blossom Drop FAQ

If the temperatures are too high or too low, the blossoms will drop.

And if you're giving the plants too much nitrogen, the plants will concentrate on producing foliage rather than fruit.

Here is a link that might be useful: Blossom drop FAQ

I was concerned about the same thing early in June when I had lots of blossoms but no fruit set. That same week, I saw that some fruit had set on my Costoluto Genovese and Cherry tomatoes. There were other plants where it seemed like no fruit was setting and I had tons of blossoms.

Fast forward one month later and it is no longer the case. Every tomato plant has fruit setting. Some with way more setting than others. The one that seems to be rather difficult is Green Zebra. I have been search and searching for that tiny, baby Green Zebra tomato and only just found one today.

We've recently had some high temperatures in San Francisco and I think that's what finally did it. I just proved to myself that tomatoes are finicky depending on the variety.

Thank you all for responding! I think the Blossom Drop FAQ explained it - we had temps in the high 90's with heat index of 105-110 for several days last week. It has been in the 80's this week and there are some new blossoms and I finally found one marble size tomato last night.
Dolores

I want to throw in my two cents. What kind of fertilizer are you using? Be sure to not use a fertilizer that is high in nitrogen. Nitrogen promotes plant growth. I use a 5-10-10 fertilizer. When you look at fertilizer the first number is always the nitrogen content. Nitrogen is great when you want a nice green lawn or shrubs; but not for tomatos. If you use a high nitrogen fertilizer you will have beautiful green tomato bushes with very few (if any) flowers.

Also, every few weeks dilute one tablespoon of Epsom Salt per gallon of water. Get it at any drug store for less than $2 for a pint size container that will last me all summer.

I got the following from the New Tribune: "Just as "Milk does a body good," Epsom Salt may be one of the most perfect nutrients for flowers and plants. And mid-to-late spring is the ideal time to nourish the soils and roots of your favorite foliage and flowers with this inexpensive and easy-to-use compound. According to the Epsom Salt Council, research indicates Epsom Salt can help seeds germinate; make plants grow bushier; produce more flowers; increase chlorophyll production; improve phosphorus and nitrogen uptake; and deter pests, including slugs and voles". Sounds good to me.

Tom