Actually, there are nothing wrong with the lighting. I took a quick look at your exif data for your pictures, and found that the under-exposed shots are mostly taken at f/11, iso 100, shutter about 1/125. If anything, there should be more than enough light to go around. Maybe you should review the way you decide your exposure and understand better how your camera metering works. Actually, correct me if I am wrong,I suspect your original pictures were more under-exposed and what you have now is after trying to tweak the exposure in Lightroom? The photos would have benefited from a moderate dose of contrast.