Hot Corners for Mission Control stop working I set the upper right corner of the screen as a Hot Corner to activate Mission Control. However, some time after setting it, the Hot Corner stops working. Keyboard shortcuts still work.

Going back into System Preferences, changing the Hot Corner to some other action and then back to Mission Control resolves the problem temporarily. But eventually it fails again.

This wasn't always a problem with Lion. Mission Control Hot Corners used to work fine. I'm not certain, but I think the problem started with the 10.7.2 update. Either that, or some other software recently installed on my Mac is causing a conflict.

Any idea on how to resolve the unreliable Hot Corners?

27" iMac Core i3, OS X 10.7.2
Had this issue November 2016 running OS 10.12.1 "Sierra" an additional symptom is that Mission Control was not launching. I fixed it by opening Activity Monitor, Quit the dock (force quit was not necessary). Went back to normal