If the blooms just turn brown and die it's because they are not getting pollinated. You can do it manually by shaking the tomato cage or the plant itself. If the blooms just disappear it sounds like the work of insects called earwigs. Try dampening a couple of sheets of newspaper and rolling into a tube. Then place the tube on the ground near the tomato plant at dusk. Check it the following morning and if you find earwigs you have the culprit. Just use the newspaper tubes at night and dispose of them in the trash the next morning.