Hmm, have you ever selected to ceate a backup point? If you dont do this, it only does it automatically corrisponding to how much space you give it in the system restore tab, and even at full, it doesn't create one that frequently, and only after a major change, like installing/un-installing a program.
To create one, you run System Restore, but select to create one and not go back. You'll find this is better and you should do this every 2 weeks or whenever you make a change worthwhile, just incase it doesn't work out, like for example a video card driver.
I hope this helps