Why doesn't itunes store recognize a movie that I downloaded as purchased?

I downloaded '2001: A Space Odyssey' directly onto my iPad some time ago. When I synced with my computer it never loaded into my iTunes library. Also since the sync I have lost the artwork for this, and many other downloads. I wanted to download '2001' onto my computer but when I go to the iTunes store it doesn't list it as purchased. I do not plan on paying for it again. Could someone please tell me how to rectify this?