why doesn't my CRT sony wega default to widescreen and fill the screen?

I have a CRT Sony Wega KV-32FQ86U. After watching or recording a movie in widescreen I am unable to get the picture to default to fill the screen (eg on BBC and the main channels ) It eventually reverts but has not done so this time. I know some programmes are broadcast in widescreen and you would expect black above and below the picture but this happens to all broadcasts on the main channels and the picture looks squashed. I have tried the zoom function but this affects writing and the picture is poorer quality.