clamshell mode doesn't work

I have just purchased a MacBook Air and am having problems initiating clamshell mode. I followed the instructions yesterday and it worked fine. Today, it doesn't work, although I'm doing the same things and have the same equipment.

I'm using a Logitech M557 Bluetooth mouse (which System Preferences can't find, although the mouse works fine), the MacBook is plugged in to an external power source, and I have changed the display under System Preferences to my Apple TV.

When I close the laptop's lid, the display on my TV (through the Apple TV) goes away as well.

Are there any suggestions as to what to do next. I apologize if there's not enough information to go on, I'm new to using Mac's. So, if you need more info, please let me know. Thanks for your time.

Donna.