iTunes purchased songs problem

[Northwestern]

I purchased some songs recently via iTunes. I saved the library also on my external HD (lucky me). After I got a new computer, I installed iTunes and checked purchased songs to redownload them. The problem is that iTunes should list ober 200 songs, but it says I only purchased about 30 songs. I checked the store pages of the albums and songs, I don't have listed anymore in my purchased list. The albums and songs are still available and iTunes shows on the store page that I have purchased them, but I can't click to redownload them. Some weeks ago, everything was fine, iTunes showed all my purchased songs and I could redownload them with one click (download all).

My question is: Why does iTunes not show all my purchased songs in the purchased area of iTunes?

Thanks in advance