Problem with Power Options

Hello Forum,

          I have a Dimension 8200, with Win XP Home sp2. In The Power Options I have my computer set only to Turn off monitor in 30 min.  I don't use hibernate, standby modes..  The problem is, is that sometimes the monitor turns off in 30 minutes and sometimes it doesn't.. Does any know why this would happen and if there is a way to fix it?           Thank you for your help..