purchased songs not listed as purchased

About a year ago, I bought a couple of songs off itunes. Recently, I wanted to buy the rest of the album. When I got to the page, I noticed that the itunes store showed no evidence of me buying them in the first place(including on my "purchased" list). I sifted through my emails and found that I had indeed purchased them, as there are receipts from itunes detailing the transactions. How do I fix this?