There are two factors in getting healthy tomato plants to fruit: environment and pollination. I don't know enough about the effects of different fertilizers or humidity on tomatoes' fertility, so I'll leave that to someone else, but growing inside does alter pollination. While tomato flowers can pollinate themselves, if the room is stagnant and bugless, you'll have a lot less pollination going on, since nothing is moving the pollen. If the flower isn't adequately pollinated, it will not make a fruit before it dies. The simple solution to this is to gently shake the plant or wiggle the flowering branches. You don't want to break it, obviously, but you want to create movement to get the pollination going; a drafty or busy room with lots of air movement can also assist pollination if you don't want to swat or shake your plants.