Tomatoes are hard to pollinate. Honey bees can not pollinate them. If you do not have other insets in your area that pollinate, then you will need to self pollinate them. Do this by taking a vibrating head tooth brush and touch it to the insides of each flower.
Tomato plants that produce flowers but no fruit have not been pollinated. A tomato is only produced when its flower has been pollinated. Reasons for this could include too few tomato plants or lack of pollinators such as bees.
It's because your Tomatoes are not ready yet! Tomato plants grow little flowers on them before the fruit comes in. These little flowers then start to slowly turn into little Tomatoes before growing to be full sized and ready to pick!