Ubook won't boot from USB thumb drive

recently my Ubook 4100 has stopped booting from USB Thumb drive. Only boots to bios when tapping F7 key while pressing power button. Both thumb drives have been used many times and booted fine. No changes to hardware. Windows 10 did do an update but that shouldn’t affect the bios.
thumb drives have Android x86 Live on them…which used to boot fine.
any ideas

Not sure why it corrected itself but it’s working now. May have been getting the sequence between power and F7 wrong.