Only two things I can thing of off-hand:
1. The CD might not be bootable. (Does it boot on other systems? What kind of CD is it?)
2. Some BIOS boot device order managers require you to hit a key during the POST to confirm that you wish to boot from the CD-ROM. If you aren't watching the screen carefully on some systems, the window of opportunity goes by very quickly. (I know from experience.)
Hope you find a solution.
Regards,
Jim