Q: Can't boot my imac G4 PPC from CD/DVD

I am trying to upgrade my 10.4.11 OS to 10.5 using a retail DVD but it just won't see the CD drive.

First it won't read the disk if I put it in it just spits it straight out again

Then I tried using the startup disk option in preferences but it doesn't list CD/DVD as an option

Then I tried holding down the option key on startup, it only gives me the HD as an option to start from

Then I tried holding down the C key

Then I tried boot cd:,\\:tbxi from openfirmware etc etc etc

So I know that you can't boot these models from a USB but is it possible they won't boot from CD/DVD drive either

Grateful for any response

Anna