iTunes Purchases

About a year ago, I purchased a song from iTunes. The song was downloaded to my device and I received confirmation of my purchase.

Now, if I wanted to back and buy the album, I should theoretically be able to do so without a) having to pay the full price (as I have already paid for one song) and b) not have to run the risk of re-downloading the same song. However my previous purchase is not recognised anymore by iTunes:

(Album with songs)

(My music library showing the existence of the song)