Mac OS X Lion won't save my security and privacy settings.

I unlocked the padlock, then the very top setting option ("Require password *drop box* after sleep or screen saver begins.") I wanted to choose "15 minutes". However, when I did so, and then clicked the lock again so the padlock was locked, closed System Preferences, then opened it again and looked at my settings, the drop box was back at immediately again. I've tried several times in different ways to change the setting, and it always goes back to "immediately". It's a pain when I walk away from my comp for three minutes and then have to punch in my password every time. On Snow Leopard, I had it set to "15 minutes", and that worked wonderfully. Can someone help me please? This is getting really annoying! I'm thinking I should have stayed with Snow Leopard!