Why doesn't my TV always switch picture shape 4:3/16:9?

I have a Sharp 76FW-53H 32" Colour TV. I receive my TV via a SKY receiver. When I switch between channels which broadcast normal (4:3) and widescreen (16:9) pictures the TV sometimes doesn't automatically switch. It means that 4:3 pictures are stretched across the whole screen, rather than being left 4:3 in the centre of the screen with black columns left and right of picture.

Why does this go wrong some of the time, but normally work properly?