Try checking for BIOS updates. I have seen older BIOS'es cause issues booting from USB drives (including newer versions adding the Boot from USB option, where it wasn't even present before).
I have also seen it simply take longer on other systems with no obvious explanation, even with newer motherboards released in the last couple of years. If a BIOS flash doesn't solve it, try what njxc500 suggested and simply wait a bit longer. There may not be anything we can do about it other than wait.