Holding down the option key isn't allowing me to select Windows/OS X on startup

Holding down the option key isn't allowing me to select Windows/OS X on
startup

I have Windows 7 installed on a 15-inch Retina MacBook Pro.
According to Apple's Boot Camp Installation and Setup Guide, pg. 10...
Select an operating system during startup
You can select which operating system to use during startup by holding
down the Option key.
This displays icons for all available startup disks and lets you override
the default setting for the startup disk that is in Startup Disk
preferences (OS X) or the Boot Camp control panel (Windows), without
changing that setting.
1 Restart your Mac and hold down the Option key until disk icons appear
onscreen.
2 Select the startup disk with the operating system you want to use, then
click the arrow beneath the icon.
I'm finding that holding down the option key has no effect -- the machine
boots into whichever OS was last running.
This is true whether I'm starting from the shut-down condition, or
restarting.
I've tried hold down each of the two option keys and I've tried holding
down both at once.
I've tried pressing the keys from before the startup chime and just after.
The only way I've been able to control which OS starts up is to restart
from the Boot Camp control panel in OS X in either OS.
Any thoughts on how to get the machine to behave as described in the docs?