application switcher - Can't use ALT-TAB to switch between windows

ALT-TAB used to work in unity on my Ubuntu 12.04 desktop. I installed gnome-session-fallback and logined using gnome classic. Now this key combination doesn't work any more.

I checked ccsm, in the "Windows Management", I do have "Static Application Switcher" enabled, binding tab includes "Tab" for "Next Window".

What's going on?

Thanks.