Some VSCode Keybindings not working in Ubuntu

I have just started with VSCode on Ubuntu and was looking for the equivalent of cmd+shift+D in Sublime on the Mac (duplicate selected text) . According to the docs I should use Ctrl+Shift+Alt+Down or Ctrl+Shift+Alt+Up but these keybindings are not working for me. When I look in File > Preferences > Keyboard Shortcuts, I see the definitions, but when I try to use them, nothing happens.