When it comes down to it, a photograph is merely a collection of tiny dots that combine to make a recognizable image.
A facial detection system attempts to find patterns in these small dots. There are a number of criteria a face detection app utilizes as it searches for faces, but one aspect it looks for which impacts on this is areas of intensity and darkness.
Extremes in lighting conditions can have an enormous effect on success in detecting faces. An area that is shadowy and dark, or an area bleached out in lightness, may include a face. However from a computer's point of view, the visible shade it sees in the image is outside its programmed accepted range of facial hues. The computer will not detect a face in this situation.