I recently had issues with a Debian installation ISO I had modified (i.e. copied out the contents, added some firmware, and generated a new ISO from) suddenly no longer being bootable from a USB stick. I ended up having to postprocess the ISO using
isohybrid, available from the
syslinux package. It added a boot sector to the beginning of the ISO.
There may have been other steps in the preparation of the snapshot, or the creation of the ISO, or the medium by which you're trying to boot the ISO that are the culprit -- plus I was dealing with an installation ISO, not an EC2 snapshot, so you may have to do way more hacking than what I'm suggesting. But at least the boot sector is easy to add:
cp /path/to/your_image.iso /path/to/your_image_with_boot_sector.iso
Some servers will treat the hybrid ISO more like a hard disk, which would require tweaking BIOS settings as well. Good luck.