For how long have the flowers been dropping off? If for only a couple of weeks, then just be patient. Meanwhile, try not to let them dry out to the extent that their leaves droop, and water them with tomato fertiliser rather than general purpose high nitrate fertiliser.

Do the flowers open before they drop off? Then it may be lack of pollination. I'm not sure whether they need to be cross-pollinated - I never do anything with mine - but you could try making sure that insects have access to the plants, and it might be worth misting with water every day or couple of days.