Coherence Issue on iMac Retina

I have version 11 and Windows 10. Coherence does not work and cannot expand window to full screen. WTH!

 

This may be worth checking. I had a problem with Coherence after upgrading to Parallels 11 on my Retina iMac. When switching to coherence a black window with a flashing white cursor appeared and the VM stopped responding. I eventually solved this by increasing the Graphics Memory (Configuration settings under Hardware, Graphics). It was set to 256MB (which is suggested as optimum by the green band adjacent to the slider) but after increasing this to 512MB the issue disappeared. I eventually set this to 1024MB as I have the 4096MB graphics card. I guess 256MB isn't enough to drive a 5K screen. Of course this setting can only be changed when the VM is shut down.