I have a Nikon D200

the auto focus works on the basis of contrast. If you aim your chosen auto focus square at an item that fills that square with only one color or shade of color light a blank wall or a solid blue sky the camera can't focus because it is trying to compare the contrast in order to focus the camera. try to make sure that you choose an item with contrast to aim your auto focus square on like someones eye and part of their cheek so the camera can see somre contrast to evaluate then focus. Obviously you depress your shutter half way to activate the auto focus let the camera do its thing keeping the shutter half way depressed move your field of vision to where you want it then fully depress the shutter to take the picture. the camera relys on seeing some contrast within your autofocus square in order to auto focus