Keyboard shortcut to lock screen stops working

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http://askubuntu.com – I've set the keyboard shortcut super+L to lock my screen. Several times I've tried to lock the screen and it just pulls up the super menu. When I go back and check the keyboard shortcut it is still set and listed as super+L. Resetting the shortcut to super+L restores functionality for a time. I can't tell what's triggering the failure. Any input would be appreciated! (HowTos)