In the tropical African countries the ground is not conducive to growing plants like wheat. PH balance is effed up.
An ancient society in the Amazon created a method in transforming similar type of earth, using charcoal, bits of pottery, etc. They were able to farm within the Amazon. There have been studies to determine the right mixture, so that they produce it.
Also, within the tropical African countries, most livestock tend to die off and are not hardy enough to contend with the myriad of diseases, insects, viruses, extreme heat, and predators.
The there is the human element---in some places where we (The west send food) warlords horde that from the people...they do that with goods as well, like gas. This was a prominant theme in the movie Viva Riva.