Producer and director Orson Welles wrote the screenplay for Citizen Kane… but he wasn't alone. He penned this masterpiece with a newspaper film critic from New York named Herman J. Mankiewicz. Mankiewicz wasn't a dude who wrote a lot of screenplays. In fact, he was usually asked by movie companies to edit and fix other people's work that just needed a little polishing.
For this reason, a lot of Mankiewicz's film work is uncredited. But he did manage to get himself a Best Writing Oscar along with Orson Welles for the screenplay to Citizen Kane in 1942. So at least Herman got some recognition at one point.