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?