The aspect ratio is fixed for HD content (we assume a 16:9 HDTV) but variable for SD content using the aspect ratio button. The default is "Auto" which uses broadcasted AFDs to determine the AR (4:3, Stretch, Zoom). By pressing the AR key on SD content, you can toggle through the available options (4:3, Stretch, Zoom) to suit the TV. Once you have adjusted the AR for the selected channel it will stay in the adjusted setting until the channel is changed, the next time you go to the channel you will need to press the AR button again if you wish to re-adjust.
Note: The TV could also have it's own AR options so make sure these are not causing issues.