If you still have the 'entries' for them on your computer's iTunes then it assumes that you still have them, even though the actual music files aren't there anymore. Try deleting one of them from your iTunes library and then see if you can re-download it - if you can then delete the other entries and re-download them as well
Ah, this just happened to me when I moved some files around. What you have to do is delete the songs from your iTunes, then go into Store and click on "Purchased" in the home page (near links for Account and Support) and select "Not on this computer". Then it should give you a link to download them again.
When you delete the song from iTunes it will say that you can only play it from the cloud. That's fine.
Sometimes it takes a few minutes, or you have to go back to the store home page and follow the Purchased link before it recognizes that the files are not on your computer.