I don't want to ask the obvious, but:

1. Are you sure you're burning a CD iso to a CD or DVD-iso to a DVD?

2. Are you using re-writable disks? I had bought a pack of CD-RW
because it was recommended for back-ups, being able to re-use them.
But I couldn't get them to burn right in any machine I own. Don't
know if it was the weird 12x speed that never seemed to be an option
in the software, but I ended up throwing out the whole lot.