Something really odd is happening with a model I’m working on. The back and top of the head remains lightless, despite my best attempts to figure it out via the various settings I’ve seen, but the top of the chest which shares the same UV map is unaffected. What would you say is going on?
I would say that you need to flip the normals on the faces where the problem exists. The light is being soaked up on the outside of those faces and traveling to the inside of the model because those faces have their surface normals pointing inward instead of outward. This is a modeling problem, not a lighting problem.
The Blender user interface has an option to “Show Normals” which creates little white lines along the normal vector.
An ordinary CG surface effectively has one side, not two. The normal-vector “points toward the front.” Specifically, it defines how … and therefore if … light bounces off the surface.
The well-known CG trick of “bump mapping,” or “normal mapping,” creates the illusion of geometry (very inexpensively …) by modifying the effective direction of the normal vector for a particular point, using a map. Because the reflections are made to go “this way and that,” as they would if bouncing off real geometry, the surface appears bumpy. Good for a golf-ball, for instance …