Nothing happens when pressing Command+R during reboot
I am trying to do a factory reset on my computer, and I have tried at least 30 times to restart and hold down the Command+R Keys and absolutely nothing happens. I have tried during different intervals during the reboot, and have tried doing it after a shut down and startup. Nothing happens at all. I have done it at least 15 times when the white screen with the apple pops up. I have waited for the "wait" symbol to try it. What am I missing?! Why won't this work? I cannot get to the point where I can erase everything. When I go in manually through the disk utility, it will not let me erase this way. Help!
MacBook, Mac OS X (10.6.8)