gnome-screenshot -- All I see is my wallpaper in the screenshot

I was just looking for a Ubuntu snipping tool and found gnome-screenshot. However, when I use Screenshot app in Ubuntu 16.04, it just takes a screenshot of my wallpaper, not of any windows -- doesn't matter if I "Grab the whole screen", "Grab the current window", or "Select area to grab" -- all I ever see is part of, or my entire, wallpaper.

Am I missing something?

This is a VM in Virtualbox, but I have 3D acceleration enabled.