Plain function keys (F1, F2, ...) and ctrl-alt-l do not work as custom hotkeys in RHEL 6

Issue

• You can create a keyboard shortcut using any of the F1-F12 keys (with no modifiers as such CTRL or ALT) via the GUI (System => Preferences => Keyboard Shortcuts), but if you use the F1-F12 keys for any Custom Shortcut, it does not do anything. But custom shortcuts work if used with a modifier, for example CTRL-F2.
• ctrl-alt-l isn't working to lock the screen.