I think I have a clue for what is happening with coherence. when I can't get the VM to switch to coherence, i check the display properties and it does not show the other monitors. It only shows the Mac display. if I click detect- it shows one unknown and you can't extend the desktop. I have 3 additional monitors connected to my macbook pro. if, while the vm is windowed, i try to switch to coherence- it fails and then the other 3 monitors appear. I can then extend the desktop on all the additional monitors and click ok to keep the changes. I can then switch to coherence mode. it appears that the additional monitors are not keeping the settings and they sometimes appear in the display properties and sometimes do not.. the times the additional displays do not appear- that is when i can't switch to coherence. hope this help to track the problem. I'm using Parallels 9- Mackbook Pro 13" late 2011. the additional monitors are connected using startech usb2vga pro (displaylink driver 2.0 and OSX 10.8.5)