I think that 6 hours of sun just isn't enough. From your description of the plant and having not fertilized, I don't think that is you problem in any way. Having nice foliage, watering isn't the problem either.

My plants get 12 hours of sun, and most are 5 ft to 6 ft tall, but this depends on the variety as well. They are loaded with blossoms and tomatoes well past golf ball size.

I think you got a "crippled" plant, but it may not be a total failure. Leave it be and see what happens.

I assume the plant was root-bound when purchased. This and other issues such as improper care and wilting in between waterings at the place of purchase (which you would not be aware of) can cause plant stresses that may affect blooms and fruit production later on. This applies to any potted vegetable. Plant may also have had a high-nitrogen fertilizer overapplied to it at the store, which it might still be benefiting from by producing lots of healthy foliage (but I personally doubt this).

Did you break up the root ball when you transplanted it?
Did you plant it deeper than it was planted in the pot?

Just curious, not doing either might have maintained a "stunting" of the plant's life cycle (obviously not an actual, observable stunting of the plant itself). Doing both would allow the plant kind of a "do-over", allowing it to refocus on growing a healthy root system and foliage in preparation for reproduction (producing blooms and fruit). Give it some more time and if you are going to fertilize, then do so with a low-nitrogen fertilizer.

Having a rootbound vegetable plant in a pot is a bad thing. It really screws up the natural cycles and processes of a plant. Proof of this can be observed, for example, in tomato and pepper starts that are already blooming and even setting fruit at the nursery, while still small-sized and potted. Those plants when subsequently transplanted will usually perform poorly, or typical expected production will be delayed. Always acquire plants as early as is realistic for you, and up-pot them into larger pots if they can not be planted out right away, planting deeper each time (tomato and pepper plants). Try to avoid buying a plant that has fruit on it - folks often think this is a sign of health but it is actually a sign of stress. Remove all early blooms when transplanting.

And absolutely yes, tomatoes want as much sun as possible. Less sunlight simply slows down the plant's processes and leads to poor results. Absolute full sun for tomatoes is best - blast 'em, they love it and will reward you. :)

Good luck,
-Tom

Thanks, Tom, for your great suggestions. Yes, it was very rootbound and I did loosen the roots and remove some of them at the bottom where they were circling around. Did not plant it deeper, though. I'm going to save your advice for future reference so as not to repeat my mistakes (there'll be new ones, of course).