I've also had trouble with two of my usb-sticks. Both of them were larger than 4GB. I'm not sure if this is just a coincidence, but if you have a 2 or 4 GB stick maybe it's worth trying.
The other idea I have for you is to set the "boot"-flag on the USB-drive:
-> plug-in your usb-drive
-> start gparted
-> choose the usb device (i.e. /dev/sdb)
-> choose "manage flags" in the "partition"-menu
-> set the "boot"-flag
-> apply your changes
-> try to boot from the usb-drive
Setting the boot-flag worked for me, when I had created a Ubuntu-liveusb that wouldn't boot...
Hope this helps
edit: disabling UEFI and going back to Legacy was also something I had to do. It helped when I had a bootable USB-drive that would start grub, but then stop and not load the OS.