Can't download purchased music

I have always bought songs on my iphone 4 (always the same apple account) and later downloaded them into my itunes library in my windows 7 laptop. Now, for some reason, I can't download them at all into my itunes library on my computer. If I purchase them on my phone, it will download into my phone but that's it. Purchasing them directly from my itunes program on my computer is the only way to have it downloaded into the computer otherwise if I purchased from my phone, it says(in itunes on my computer) "purchased" in dull grey lettering but without the download symbol that used to be by it. I go to the store and find the items I bought but I can't download. I think this started happening a few months ago but I'm not sure. I have everything updated on my computer. How can I fix this? Am I dong something wrong?