in Ubuntu, when there’s menu active or visible, pushing ‘Print Screen’ won’t have any effect. It applies to taskbar, context menus, right-click and other menus. But I have a workaround on this one:
Simply let Gnome automatically take the screenshot, and delay the action.
In your terminal, type:
gnome-screenshot --delay=5

The number 5 represents the number of seconds before Gnome automatically take the screenshot. After typing the command, go to the window or menu that you would like to take the screenshot of.