Help with collections

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This should work, but is all based on metadata. If you add a movie, it needs to have the exact same collection name in order to be added to the same collection. If you're relying on disparate sources or Jellyfin's scraping capabilities you might be disappointed. I would point you toward examining the metadata of a couple of the films that are already in the collection -- specifically TMDB Box Set ID (I'm sort of guessing here, but this is the only ID that corresponds to "collection" I can find which matches in my collections).

My guess is that the odd film out doesn't share the same box set ID and thus doesn't qualify to be added to the collection automatically. You might be able to fix by refreshing the metadata on the film.

EDIT: You can also try the plugin that helps with this, but I'm unsure what using both the built-in setting and this plugin will result in...play around at your own risk: https://github.com/jellyfin/jellyfin-plugin-tmdbboxsets.