Why isn't my city growing?

It is my first game of Civilization 5, but I have played Civilization 3 and 4 many times. Twice I have seen my capital having a stagnation for no apparent reason. From what I read food is the only reason a city would stagnate. I have even seen a city with more feed than required being in stagnation.

At this moment I have a source of 15 food and 12 are consumed by civilians, so this makes 3 extra food. As I understand it, there is no reason there should be a stagnation, but there is. I guess there is also a happiness factor, but my happiness is well above unhappiness.

What could cause that? Could it be a bug?

Whenever you build a Settler, all your extra food goes towards increasing the production of that unit. Due to this your city will always be stagnant when you're building a settler, and I'm willing to bet this is what is happening to you.

There is an "Avoid growth" option, which prevents city from jumping to the next growth level. Maybe you selected it without knowing

The option is on your right panel, right at bottom of Citizen Management options.

See full detail here: http://i.stack.imgur.com/pQ85c.jpg

Even though the city has enough food to grow it won't grow.