Hasn't the weather in LA been really warm this year. If your plant is of a large variety of tomato the low temperatures have been too high for them to set fruit. I could tell more about your situation if only you would have sent a couple of pictures.
Heat and humidity will keep tomatoes from setting fruit. The pollen gets sticky and won't fall onto other flowers, and the heat makes the plant "shut down" until cooler weather.
Believe me, here in Texas we understand heat and humidity. I take this opportunity to trim back our tomatoes by 1/3 to 1/2 and keep them watered enough to stay alive. Then in the fall we get another crop of tomatoes, smaller than the spring crop, but welcome nonetheless.
If it's temperature which is the problem, then the plant will be producing flowers which don't set fruit and drop off.
Since you don't mention this, I'm going to suggest it's too much nitrogen.
Get half a trowelful of woodash, if you can, and work it around each plant in a one foot radius, then get woodchips and mulch to 3 inches, again in a one foot radius around each plant. Liquid feed with seaweed or comfrey.
The woodchips will sequester some of the excess nitrogen as they start to rot down, the woodash and liquid feed will provide the potash for fruit growth and the minerals for flavour.
The woodash is alkaline, but the decomposition of the woodchips is an acidic process.
Same thing happened to us, this year.......BIG plants, very few blooms......very few tomatoes (MAYBE a total of 20 from12 plants).......except with the grape tomatoes....and the skins on them were so tough you, just about, couldn't eat them......