My Spaces hotkeys have just stopped working

This started a few months back. I have the Spaces hotkey mapped to F12 and to one of the alternate buttons on my mouse. Every now and then, for reasons I cannot even begin to divine, both hotkeys just stop working. I can still switch with ctrl-arrow or by hitting the corners, but just the hotkeys to pull up all spaces stop working. I can only fix this by resetting the entire computer, and even then that doesn't always work. Does anyone have any possible idea why this might be happening? Thank you!