what adapter are you using ? If you read around some people are having issues with non samsung HDMI MHL adapters. Are you on a rom ?
Samsung's MHL adapters (11 pin) allow the simultaneous use of USB and HDMI unlike the rest of the MHL crowd (5 pin). They sorta said screw this standard...it's weak. We'll do our own thing.
So yes, if you have a genuine Samsung MHL adapter, and it still doesn't work, then you need to investigate further. I've even read that Samsung allows the phone's battery to power the MHL adapter, and a micro USB is not a requirement.