[G4] G4 won't read OS X upgrade CDs

[David Minger NPG Cable]

Hi,

I've got a whole range of Macs from 68K up through G5s. I have 2 G4s  
(a 450 and a 533). I've been able to keep the 450 updated to the  
latest OS X, but the 533 is stuck a couple of feline animals into the  
past. For whatever reason it will not handle the upgrade CDs that the  
450 handles just fine.

The 533 OS is therefore getting more and more out of date.

Any ideas on what the problem is and how to fix?

Best,
David