Its stumped me!!!

[Beryltheperyl]

I don't know if this is the right forum for my question (Please don't hit me if its not), but I've got an internally chipped PSX, and my friend as got a ###ne with a disc swap. Now some of the back-ups work on mine, but don't work on hers (either the screen freezes, or doesn't start at all). I don't understand it, if they were bad copies, surely they wouldn't work on mine either? Any1 come across this b4??