If I've focused any application or my mouse is over launcher or top panel, dashboard works just fine. But when my mouse is on the desktop background, dashboard does not show on super-key-tap. What I've tried:
unity --replace - no result
disable wallch - no result
changing wallpapers manually - no result
logging off - no result
rebooting whole system - no result
This is second time I've experienced such a problem, last time I had to reinstall my whole system, but it was when I was tweaking the Nvidia drivers.
Running on Ubuntu 12.10 64 bit with nVidia GeForce 635M on Unity DE.