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It sounds like you want to re download the Avengers movie from iTunes.
You should be able to use this process to download the movie again:
- Click iTunes Store near the top of the iTunes window.
- Click the Purchased link at the right of the iTunes Store homepage (below Quick Links).
- In the page that appears, select a category.iTunes shows you which of your purchases are available for download. You can view all of your past purchases, or just the ones not currently in your iTunes library. Items already in your library aren’t available for download.
From: iTunes 11 for Mac: Download previous purchases from the iTunes Store
If this is the process you have tried I would reach out to the iTunes Store support with this article:
How to report an issue with your iTunes Store, App Store, Mac App Store, or iBooks Store purchase
All the very best,