I noticed this when I went out early in the morning to take some landscape photography that sometimes when the image was "dark", the camera would not attempt to take the photo. I tried changing from a number of different modes (manual, auto, landscape, TV, etc etc) however it seemed that because the camera was unable to focus well on the target or wasn't positive that the camera was focusing onto the target that it would then attempt to focus, fail and give up with capturing.
Is there a reason for this? I had manually focused using a focus ring and had the issue on a standard 18-55mm kit lens as well as 70-200mm f/2.8 L IS II USM lens. Is it because of the camera model itself or a setting within the camera?