Farms only produce food to your population if they are "worked"
This means one of those 9 citizens is working on it, perhaps they are set wrongly and producing poor food. Another problem could be polution, each excess polution consumes 1 food. In this case there is a green haze over your city, if that is the case try and increase public health by building improvements such as Aquaducts, Hospitals and try and get a health bonus from curtain resources, most of them are food such as wheat, crabs, fish etc etc.
