OK....
Assuming a correctly constructed USB install,
And assuming the BIOS gives the option of more than one 'thing' in the boot order (as do most).
....put ALL options in the BIOS.
The way it works is...if it fails to see anything at Option 1, it goes to Option 2. Still nothing? Option 3.
and on....
Try that, with the USB stick in a USB 2.0 port.