I had a similar problem which I solved from a help support email. I've pasted the info below which may be of help:
"Fortunately, the iTunes Store gives you the ability to download your past purchases without incurring additional charges as long as the items are on your Purchased page. You may be able to use this feature to download your purchase(s).
To learn how to take advantage of this feature, visit:
Downloading past purchases from the App Store, iBookstore, and iTunes Store
If what you want to download is not on the Purchased page, it may have been removed or modified, and no longer available.
You can also check your hard disk for unavailable items. Just follow the steps in this article:
iTunes: Finding lost media downloads
If you have items on your iPhone, iPad, or iPod, you may use iTunes to transfer them to your computer. This article will walk you through that:
iTunes Store: Transferring purchases from your iPhone, iPad, or iPod to a computer