Are the blossoms dropping? Like, the flowers appear but then after a few days, they just fall off leaving sad little stems? If that's what's happening, then your flowers aren't getting pollinated. You can try dusting over the flowers with a feather or planting some marigolds near your tomatoes.
The fruit themselves will 'set' right inside what used to be the flower, but it will take a while for you to be able to see that this has happened.