I'd report the problem to the iTunes Store.
Log in to the Store. Click on "Account" in your Quick Links. When you're in your Account information screen, go down to Purchase History and click "See all".
Find the item that is not downloading properly. If you can't see "Report a Problem" next to the entry, click the "Report a problem" button. Now click the "Report a Problem" link next to the item.
(Not entirely sure what happens after you click that link, but fingers crossed it should be relatively straightforward.)