A fairly common problem with tomatoes can be a condition called Blossom Drop. It can be caused by a host of environmental conditions such as the following: too hot or too cold temperatures, lack of pollinating insects, or too much nitrogen. You can't control the weather, but be sure you don't overfeed your plants. Generally, this problem fixes itself over time. You haven't give me enough information to really try to nail down your particular problem, but I hope this helps."