He was removed from the subversive file ONE day before the USSS searched it before the parade route. They always search the file before a parade.
Had this system worked as intended than USSS would have shut down the Book Depository and would have held Oswald in custody for the day. Even Hoover himself remarked how unconscionable this all was and he punished several agents for it.
This _particular_ lone nut should have been EASY to stop.