There can be many causes of famine, ranging from natural disasters to human actions such as wars. The immediate causes of the famine tend to be insufficient food production and/or poor distribution of food.
Insufficient food production tends to be related to a lack of technology. In places where farmers are unable to produce enough food, it is because they lack good seeds or chemicals or irrigation or other forms of technology that make modern farmers capable of producing such huge amounts of food.
Poor food distribution can be an economic problem or a political one. There are places where food distribution is hampered (North Korea, Somalia) because those in power do not wish the food to be distributed. There are also places that lack the resources to build roads and such to easily distribute food to the people who need it.
Please follow the link below for a more in-depth look at causes of famine.