Your soil could be the issue. All blooming vegie plants need a boost and regular household Epsom salts is what to run and get. Google it, go to their site, many online planting sites will give you info. Even on the packaging it tells you how to use it.
Helps with bottom end rot as well. Magnesium Sulfate is needed in the soil as a boost. Do you get your soil evaluated/tested so you know what is in it or do you buy planting soil at the nursery/farm store, etc. I have my own mix and have found that Epsom salts also helps control bugs in my gardens too. Don't let it set on the plant, sprinkle around the base and water. My roma plants had 100 tomatoes to a plant last year. Could not believe it! Did the Epsom salts about every 2 weeks.
Are they all the same variety? There is determinate tomatoes and indeterminate varieties. Some flower sooner than others. Also, I use tomato fertilizer on mine and I think it helps. The tomato fertilizer is also good for clematis if you have any of those as the same ingredients in it is what you need for clematis. (05/12/2010)
I think it's a nutritional deficiency. Go to garden section and read on the smaller boxes of fertilizers that say, increases blooming, and find out what you are lacking. Then you can buy ready made or go organic route and add what's needed. (05/12/2010)