I downloaded a digital copy of a movie on iTunes but the extras had a download error. How do I fix this?

I had downloaded a Digital Copy of a film with extras on iTunes the other day. The movie is working fine, the extras met with downloading errors and won't work. I've tried clicking the download button but keep getting the same error message. I've already redeemed the code so I can't do the thing all over again! Please help me because I want to see those extras!