September is the 9th month correct. The inauguration was on January 20th and the attack was on September 11. That is less than 8 month. Besides the warning which was unspecific came six weeks before the attack and you wanted him to do what William Jefferson Clinton failed to do?
With the state of the intel he was getting at that point, the threat likely appeared to be nothing greater than a shoe being thrown at the President during a presser.