Thanks for writing. Are your vines producing blossoms? If so, then it sounds like the tomatoes are either aborting (which can happen from stress --- too much heat, too little moisture, etc.) or not being pollinated.
If the plants aren't producing flowers at all, then there's probably some sort of stress on the plant. Make sure your tomatoes are in full sun and moist, well-drained soil for best results. They're heavy feeders, but use a balanced product to fertilize them as too much nitrogen can encourage lots of leafy growth at the expense of flowers.
---Justin, Senior Garden Editor, BHG.com