I have attached an external monitor to my 2014 macbook pro retina display. I have connected through the dvi port on monitor via a dvi to thunderbolt adaptor and plugged into my thunderbolt port. The problem is I am not getting any signal to the external monitor. Am I setting this up incorrectly, I have only found things saying use a thunderbolt to hdmi cable. Not sure if that is what I need or not sure if the monitor I have is compatible, the monitor is only roughly 3 years old it self. Any help much appreciated.