Er, that's not an easy one to answer.
I have had problems that once I get to six or seven cities I remember I need to set the governor to emphasize production. I do that in the capital and check the box 'default for new cities', which is fine for all the future cities. However, I forget to go back and reset each of the current cities. And I don't recognize my oversight until a settler isn't produced on the turn that the city grows.
Could this be your situation, too?