I was stuck on this for a very long time. I couldn't figure out why things would show I purchsed but I couldn't download. Found out the songs and albums were hidden. So here's what I did to fix my issue, hopefully it will help people who come to this link and need help like I did.
1) Go to the iTunes store
2) Click on "Account" in the quick links (right side of home page)
3) Under iTunes in the Cloud, look for Hidden Purchases, then click on the link to the right that says "Manage"
4) It should show all of your "hidden" songs here. Simply click the "Unhide" button under each album
5) Go back to the iTunes Store home
6) Click on "Purchased" link in the Quick Links
All the albums that you couldn't see and download before should be there now. Hopefully this solves anybody elses issues like it did mine with this. I had to read through several forums before I found this out. Good luck all!