If the image is not appearing on both the laptop and projector, press one of the following key combinations: [Fn] + [F4] for HP's or [Fn] + [F8] for Dells or use the "Connect to projector" option located in the start menu on your desktop.
MacsThis combination will toggle between displaying the laptop only, the projector only, or both the laptop and the projector.
Wait 5-10 seconds between switching modes.
SYSTEM PREFERENCES > DISPLAY > 'Detect Display'
Make sure "Mirror Displays" is checked in the 'Arrangement' tab.