My install ISO won't boot on USB stick

Okay, I've never seen this before. I have the 7.4 ISO - both the Boot image and the full image. (I've tried both.) I restore the ISO image to my USB, reboot my machine, go into BIOS, direct it at the USB, and nothing happens. It just went back to the BIOS screen, nada.

The USB had a GPT partition table with one partition that the ISO image was restored to. I blew that away and created a MSDOS FAT partition, and restore the image that way. This time, it appeared to recognize it as a boot device, but the cursor just sat there blinking, zip.

One more try - - - keep the MSDOS partition table, format the partition as ext4, and try again. Nope - same thing that happened with FAT.

I'm at a loss. I've tried Legacy and UEFI boot modes. I do have secure boot enabled and I can't un-enable it. I've tried loading keys into my BIOS too - no dice.

This same server is being upgraded from CentOS. I've installed at least three versions of CentOS on this same server if that helps at all.

I don't suppose anyone has any ideas? Maybe try the 7.3?

Thank you and happy Saturday!