RCA HDTV. Sound, but No Picture. Weird.

The sound and picture on my tv work fine when I view high defintion content using the composite inputs and a Microsoft Xbox hooked up to them. However I cannot view anything hooked up to the coaxial input or component inputs. The high defintion inputs will also not display anything that is not high defintion. Through all of this the sound works perfect though. Seems that only HD digital signals will display, everything else is a no go...