I recently had a personal experience with this that gave me some insight into one other possible cause of the "won't boot
from a bootable USB drive" problem. Since these posts are spread all over time and various forums, I decided that this is
the best place to throw this out.
Several years ago, many (most? all?) USB flash drive manufacturers made a change to the firmware on their USB flash drives
that caused them to report themselves to the PC not as removable drives, but as hard drives (or as SSDs, if you prefer to
think of it that way). I forget their rationale for doing this, but recall there was a lot of consternation over this
decision, because it would block these newer flash drives from being used as bootable removable drives (ruh-roh).
Consequently, a PC wouldn't be able to boot from these USB flash drives just by setting the boot priority to the removable
drive, no matter how perfectly you follow the directions in any article on this topic to make a flash drive bootable.
This drove me crazy for several days while trying to do a Windows 10 clean install on a new SSD on my PC. I tried three
different USB flash drives (a Lexar, a SanDisk, and a Verbatim), but none would boot, despite the fact that a fourth USB
flash drive (another SanDisk) that was set up as bootable (for a hard drive maintenance utility I use) would boot up the PC
just as expected (by setting the boot priority to "Removable drive" in the BIOS). As I was staring at my four flash drives,
it suddenly occurred to me that the one that would boot was about 10 years old, while the other three were all less than 2
years old. That's when I remembered what the USB flash drive manufacturers had done several years ago.
My solution came courtesy of the AMI BIOS on my 2010 PC (so YMMV), which allows me to set not only a device type boot
preference order (among hard drive, removable drive, and optical drive), but also a hard drive boot preference order.
Opening that latter item in the Boot section of the BIOS (with one of the bootable USB flash drives inserted) displays a
list of all my hard drives, including the bootable USB flash drive (because it's reporting to the BIOS that it's a hard
drive, of course). By setting the USB flash drive as "Hard drive 1," it became the primary boot drive, enabling me to boot
up to the Windows 10 installer and perform the clean install on my SSD. As it turns out, this is also the only way I can
use one of my newer USB flash drives as a Windows 10 Recovery Drive (but it does work for that purpose).
As a side note, I'm also able to use this BIOS feature as a boot manager by selecting whether my old Windows 7 system drive
or my new Windows 10 is "Hard drive 1." It's a little crude, but it will get me by until I've gotten Windows 10 configured
to my satisfaction, at which time I'll switch over to using Windows 10 full time.