My macbook pro, c2d 2006 model, refuse to shut down when I want to, most of times. I want to shutdown using the apple menu, doing it the right way. It displays only the backdrop image, I cannot see any circle in the middle of the screen showing me that it is going to shutdown, and thats all. I have to press power button for some seconds in order to shutdown. What can I do to find out what prevents mbp from shutting down?