Thomson W/S Auto aspect ratio switching

When I first bought this Thomson 28WF45UG just before Christmas, I'm sure it used to switch from 4:3 to 16:9 automatically, i.e the picture would appear in 4:3 for a few seconds at power up and then switch to 16:9 (or 14:9 whatever). Now it doesn't do this and I have to switch to 16:9 manually by going through the menu , which is a bit of a pain. It does say in the manual that when changing channels, switching will revert to Auto. My question for the experts is: Has the auto switching somehow become defunct, or could it be anything to do with the fact that the RF is "daisy chained" through a DVD recorder, and a VCR before it appears at the RF input on the TV ? A Sky Digibox is in the configuration, but is connected via a Scart. Thanks in advance.