The Mystery board says a clue was "Not found" in its chapter. What should I do?

From version 1.9.22, "Check my chapters" marks a planned clue as "Not found" when it could not find it in the chapter where you placed it. Either regenerate that chapter from the book's page (the board's clues for that chapter are given to the AI again; the old text stays in the chapter's Versions), or open the chapter in the editor and add the clue yourself, in a sentence that does not draw attention to it. Then run "Check my chapters" again. Fair play on the board also reminds you of clues that were not found.