downloading a movie

Did you buy from the Apple iTunes Store (Store tab)?

If you made the purchase on iPad, then it would show in the TV app under (Sidebar)﹥Library﹥Movies﹥.

It would also show on any of your other devices with the Apple TV app, when signed in with the matching Apple ID account.

If it shows that you can buy it again, then your previous buy attempt wasn’t successful. It is not possible to buy the same movie item twice on the same account. Already purchased items would show a Play button (or Play Again, or Resume) in the Store, instead of a Buy button or Rent button.

Does the purchase show in your purchase history?

in 2015.

Ok. Could it be that this title was set to hidden at one point? Then see if you can unhide it (on a computer).

Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support

Else please contact the iTunes Store staff, as they are in the best position to investigate this further.