Too much high-nitrogen fertilizer (the first of the three numbers of the package label) will promote vegetative growth at the expense of reproductive (flowers, fruit) growth. Try using a fertilizer that is specifically for tomatoes, or at least for fruiting plants - it should have a formula higher in P and K (the 2nd and 3rd numbers). Not enough sunlight will do that, too, tomatoes should have at least 6 hours of sun per day. Another possible cause is heat stress - if it is routinely above about 90 in the day, and doesn't cool down much below 72-75 at night, many tomato varieties will have flower bud abscission or if the flowers open, the fruit doesn't set. Finally, sometimes chipmunks will take a liking to flower buds of certain plants - the plants send a lot of sugars into the developing bud to help the flower grow and form fruit, and that makes them taste sweet. I've personally had that problem on impatiens, where the chipmunks strip the flower buds - don't know if that happens with tomatoes but I could see them doing that.