Check the Navigate ▸ Move Focus To ▸ Header Bar Title menu command and see if the shortcut is correctly printed there, and that the menu command isn’t disabled (indicating the cursor is currently in an area that can’t target the header bar). If it all looks like it should be working, then the interference is probably coming from another program setting a global shortcut. Those can be hard to track down, especially if it isn’t listed in the System Preferences: Keyboard: Shortcuts pane somewhere. You just have to go through preferences in the stuff you have running.