Heidi, just had another couple of thoughts after reading Rick's suggestions.
First, have you (or anybody else like a previous owner) grown tomatoes in that bed before? Soil-borne pathogens can persist in soil from year to year, especially in places like California where you don't get freezing weather to kill stuff off. Crop rotation is a very good practice in that case, i.e. don't grow tomatoes in the same bed two years in a row. Or here we solarize the soil by covering it with clear plastic for about 6 weeks to 'bake' it in the sun. No chemicals involved.
http://gardeningsolutions.ifas...
Second, you could test your soil's pH levels and soluble salts with a simple kit available at the big box stores or nursery. It's about $10 for ten tests, or you can get a single use one for under $4. As Rick said, test the soil where the plants are ok, and also where the plants are in trouble to see if there's a difference.
