External display no longer works in new MBP

I have this Acer h233h display, and it's great. I've used it on my mini and macbook without any problems. However, I've just purchased one of the new i5 macbook pros, but when I try using the external display, nothing shows up. The lcd will cycle connection ports, but display nothing. Every once in a while I'll get it to display, but once I disconnect, or restart the notebook, it won't work anymore. I'm using HDMI.

Any ideas?

Thanks!