My new 2013 13 inch retina MBP has stopped working with my external monitor. It worked last week, then only on one TB port and now neither works. When I plug mini display port cable into the thunderbolt port the Macbook screen flicks off/on like it normally does, but the monitor detects no signal. the same cable and monitor work fine with my 2012 MBP (not retina). Anyone have any idea why this is happening and what I can do to fix it?