I've had that issue in the past with some motherboards just not liking my USB keyboard, or ignoring wireless keyboards for getting in to BIOS. The only other option I can think of is try the key that allows you to select the boot device (usually called a BBS). It enters in a bit after the initial boot of the bios but before the OS is loaded and a lot of times (depending on the bios) an option to go in to setup. On Asus its the F8 key, but your board may be different.
Other option is make sure you have your keyboard plugged in to a USB 2.0 slot. Some boards will even designate the USB 2.0 ports by the PS2 ports specifically for keyboards/mouse for this very reason.