On the units that work, is the screen display the older firmware "brown" pattern or is it the newer "gray" look? About the time they made the switch in firmware that changed the "look" of the screen, they also added an FSC code for activation of some features. The FSC code is VIN specific.
I'm just shooting in the dark with this, but it might be part of the cause. I've got a DVD with all the US VINs and FSC codes for CCC units somewhere.