Comment 22 for bug 1313539

I had this both on Elementary OS and Xubuntu. I've got an AMD card and tried using additional drivers, so I selected one of the video drivers, in my case I picked the following option in eOS 'Using Video driver for the AMD graphics accelerators from fglrx-updates (proprietary)'.

This solved the problem of not being able to consistently use a secondary monitor which was using a display port. However I'm now not able to screenshot anything as it only shows the wallpaper now.