Bin Apple and bin ITunes,
Unnecessary Apple bashing... ignore him.
If its in your iTunes purchase history, it should be feasible to download it again.
How old are your purchases ? I know in the first few years around the launch of iTunes, Apple were forced by the record companies to be more restrictive in their policies. Their current policies are far more relaxed, I believe they have retrospectively applied these policies to old purchases, but I'm not sure.... it may just be a case they need to update a few things on their system for some of your older purchases.