To elaborate on what Vieuxchat is saying. You have a limited number of "tiles" that you can put buildings on. Once you place a building on a tile, that tile is developed. You can fill that tile (i.e adjacent buildings) but you can't actually open up another tile unless you have the room.
So basically... if you can't build more titles... wait a few turns for your town to "upgrade" and you'll be able to build anywhere again.
