My tomatoes aren't growing.?

I have a fairly large tomato plant and 4 other smaller ones. The large one has had flowers for about 3 weeks, but they never grew tomatoes. Some are now getting to the point where the whole flower and green leaves at their base are just falling off leaving nothing but a nub where the flower once was. Any suggestions on what I could try, I keep them in a fairly sunny area and well watered. They are baby tomatoes if that makes a difference. Thanks