Yep, the problem you're having is due to not having enough sunlight. Petunias need full sun to maintain that beautiful flush of blooms, you won't get any until the plant gets more sun. Good luck! Pigletpip

90% sure that the lack of sunlight's the problem- they're not just sun-loving, they're totally sun dependant. that's why you see them so often as mass planting in the middle of parking lots and golf courses.- they're drought tolerant, and sun-drinking.

It also helps if you pinch off the stems of spent blossoms and not just the flowers. They tend to go to seed if you aren't careful.