Downloading a previous purchase

I purchased an album a few years ago that now will not download in it's entirety into my iTunes library. The album is still available in the iTunes Store, and my purchase history still indicates that I have purchased the album. When I select the "Show Complete Album" option in iTunes, it just says, "No additional songs found", even though there are loads more songs on the album, all of which are listed in the iTunes Store. I've tried using my PC, iPhone and iPad, and the problem is the same on all devices. How do I get my missing tracks?