iTunes pre-order not completed

Hi

I recently pre-ordered an album on iTunes and the tracks that had already been released were automatically downloaded. The album has now been released in full but the remaining tracks are not available for download. I checked my Purchase History and have only been charged for the tracks downloaded at the time of purchase. However, I am unable to purchase the remaining tracks to complete the album, as iTunes says I have already purchased them.

Any idea how I can resolve this?

Many thanks.