Flag/Unflag shortcut in mail.app

I learned that ⇧⌘L in mail.app should toggle the flag of a selected message. However, it doesn't work for me... I'm using Snow Leopard and Mail 4.5. Any ideas?

In Lion the shortcut is exactly that and you can actually take a look at the menu:

Make sure you're using the right one. If you wanted to alter those, you'd have to go to System Preferences -> Keyboard -> Keyboard Shortcuts and add a new one:

cmd-L was set as the input source shortcut. This caused a conflict with the cmd-shift-L of mail.app. Once I have changes the input source shortcut (to cmd-M) and restarted mail.app everything worked as expected.