D5100: When i point to a wall, and use shutter release button, it just tries to focus and that is it?why so?
Most likely the camera is failing to achieve focus (since it doesn't have anything with contrast to determine if it is in focus or not). Some cameras will either not take a photo when they can't achieve a focus lock or have a setting that will tell them not to take a photo when focus lock can not be achieved.
It sounds like your camera either behaves this way or has a setting to behave this way turned on. I unfortunately don't know enough about your specific camera to know if it is a behavior you can change or not on your particular model, but it is typically called something like Focus Priority.