rMBP '15 mid 2014 - dark display in clamshell mode

Hi all

I have rMBP '15 mid 2014 connected to BenQ UHD BL2711U monitor via DispPlay Port 1.2. High Sierra installed.

All works well while the MacBook is open. As soon as I close the lid, the external display goes dark (not to sleep). I can put it to sleep via keyboard shortcut. But when I wake it up, the display goes back to dark dimmed state. The power is connected to the MBP and wireless keyboard and mouse all function.. MacBook is not outputting to the external display in 'closed lid' mode. Any thoughts?

thanks