"Growing tomatoes in a greenhouse has many pitfalls. Make sure the plants get as much sun as possible and lots and lots of water. Control temperature and humidity and keep careful watch on insects and diseases. Also, the flowers may not be getting pollinated in the greenhouse because there are no insects. That could cause the fruit to not set. You'll have to hand pollinate to get good fruiting.
There's a tomato variety out there called 4th of July. It's the earliest variety I've grown. You get 2 to 3-inch fruits, but they are juicy and tasty.