Restore Point problem

Using restore points on the  XP I find to be one excellent feature. Being able to go back before an install of a problem program or malfuncting computer function saves doing a complete Systematic Restore. But lately after doing several restores I finally run into a blank wall where no restore point will work. This has happened to me twice now and I wonder if there is a limit to how many times this feature can be used. I love to experiment with new programs and then if I am not satisfied use a restore point to "eliminate" them or the computer function it caused. I must now do a complete restore. When you do this you always have a lot of baggage to eliminate and software and updates to load so the restore point following this point saves much time rather than a complete restore but only when the restore points work. What could cause this problem?

 

Carroll