Retina MBP - external monitor not working

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?

 

No one else having the same problem?

Update:

I spoke to applecare, reset system management controller and external monitor worked. 10 minutes later I had to disconnect MBP then tried to plug into external monitor again and it failed again.

I can't reset the SMC all the time. I'm going to give the Genius Bar one chance to fix this and then ask for a replacement computer. Its 2 months old, but I'd expect this to work and I don't want them tearing out the logic board and TB ports to replace them to see if that fixes it.

Hopefully the store techs can find a solution. Is no one else having this problem though? If not, I suspect there is a problem with my particular laptop.