You are doing something wrong. Actually, you are doing plenty wrong. Creating a bootable cd or dvd is somewhat of a black art but suffice it to say you have to do more than stick some files in an iso and burn the iso. You also have to properly configure a bootloader, usually isolinux, and properly generate a bootable iso with mkisofs. Here's a little intro to give you an idea about what's involved:
http://syslinux.zytor.com/iso.php
I assume you are trying to consolidate several install cd isos into a single bootable dvd iso. It's theoretically possible but it isn't all that easy. Labor and effort wise, you are usually better off just burning the cds unless you are very familiar with the cd layouts in question, iso manipulation and various arcane aspects of isolinux and mkisofs.
