Several times it proved not easy to establish a solid connection between a Macbook (Air and Pro) and an external HDMI screen or projector. I used several brands of adaptors, no difference. Lately I heard the same complaints in my network. Rocking a bit with the thunderbolt connector until the image appears mostly soves the problem. It lookes like fiddling araound with the utmost flimsy Chinese junk. I wonder if Apple does know about it.