Many laptop computers require a function key or software command to activate or deactivate the video output signal from the back of the computer. Most laptops allow you to use the activation/deactivation command to toggle through the display states. First you choose the command to display the image on the laptop screen; repeat the command to view the external display (projector); and repeat the command again to display both simultaneously.
Contact your laptop manufacturer for the function key or software command for your laptop.