'show mail' keyboard shortcut works intermittently

Seemingly intermittent problems are the worst! The cmd-opt-ctl-D shortcut to show and hide the mailboxes works for me sometimes, but more often not. But if I click View and hover the mouse over Layout, then escape and use the shortcut (obviously not a practical shortcut), it works fine, and will continue to work if I repeat the keyboard command. But after various activities, probably with MM in background, the shortcut stops working. I haven't found a pattern beyond that.