This problem has been bugging me for a very long time--custom and media keyboard shortcuts occasionally not working; logging out and back in usually fixes it. Comment #63 fixed it for me immediately (did not have to log out). :-) To be clear, I ran CompizConfig (search for ccsm) and, under the "General" category, unchecked the checkbox next to "Commands". What's more, this same procedure fixed the PrintScreen button on another computer (both Ubuntu 14.04.2). I have not tried re-enabling CCSM Commands.
This problem has been bugging me for a very long time--custom and media keyboard shortcuts occasionally not working; logging out and back in usually fixes it. Comment #63 fixed it for me immediately (did not have to log out). :-) To be clear, I ran CompizConfig (search for ccsm) and, under the "General" category, unchecked the checkbox next to "Commands". What's more, this same procedure fixed the PrintScreen button on another computer (both Ubuntu 14.04.2). I have not tried re-enabling CCSM Commands.