Do you mean it doesn't go to sleep at all, or some other amount of time than 30 minutes. How about turning off the screen effect until you get this sorted out, you can always set the screen saver to start by moving your mouse to a corner. Also, 30 minutes means you have to leave the system idle for that length of time. But try changing the Energy Saver pref to the shortest time for a test, or turn automatically go to sleep completely off, and just do it manually when you leave the computer, there's lots of options here. (does your display have a setting for this in energy saver options?)