PS2 not recognising PS1 game as valid disc

[OzCapital]

I have an imported NTSC PS1 game that is not recognised by my PAL PS2. It simply says "Please insert a Playstation or PS2 formated disc". I have created a patched PAL version of the game but I get the same message. Both the NTSC and patched PAL version run fine on a PC Playstation emulator so there is something about the disc that makes my PS2 not release it really is a PS game. Does anybody have any idea why this might be the case. I'm not at all sure if it has anything to do with the video system. Is there some general thing about US games that they don't run on PAL PS2? Is there someone who is more experienced with PS1 imports running on PS2?

thanks,
Adam.