Generating PAL tv signal using a PIC
It could be a SCART switching issue. With SCART, some TVs need a low signal on pin 16 to select RGB rather than composite.

http://freespace.virgin.net/matt.waite/resource/av/scart.htm#Note%202%20:

You could also try putting your composite signal into one of the RGB channels to see if it's picked up properly.