Why do you need to restore? If it won't let you then you will need to look at other ways to fix your problem.
System Restore doesn't always work. Depends on what the original problem was.
My System Restore NEVER works and hasn't in quite a while. If I really get in severe trouble I have to use Acronis to completely restore my C drive as opposed to simply restoring it to an earlier good state. I recently had this situation with some malware that disabled my anti-virus and froze up my computer. Acronis saved the day, but System Restore would have been better.
Have you tried manually setting a restore point every couple of days to see if that works? Usually a restore point will only be made when new programs are installed. If you don't do any installs then you won't get new restore points.
By the way, virus' and malware can infect the restore points also.