I am tuned to an HD channel, however the picture is not displayed in full screen.

Not all programs are broadcast in high-definition and therefore programs will not always be full screen. Not every show currently broadcast by an HD network is in HD format. If you are getting a picture on the HD channel, you can assume that your equipment is functioning properly. You may want to check the settings on your HDTV set (consult your HDTV user manual).