One of my chapters is still marked "short". What can I do to make it longer?

There are a few ways. First, regenerate the chapter: hover its row on the book's page and click the Regenerate icon (the current text stays in its Versions); the AI often reaches the length on a new try. Second, choose another model: some models stop early whatever they are asked. Change the engine in Book details, "AI engine for this book", or the model for that engine in Settings, then regenerate the chapter. Third, lengthen a passage yourself: open the chapter in the editor, select a passage and press "Expand" in the "Rewrite with AI" card. If the model you like always stops short of long chapters, choose a lower "Words per chapter" when you create your next book in New Book (on the Structure step) and plan more chapters instead.