song authorization

I recently downloaded some songs from itunes on my laptop. Some of them worked fine, but others did not. The ones that did not work said that this computer is not authorized to play this song. When i try to authorize the computer it says that it is already authorized. it does the same thing on my desk top at home, and it will not let me download them to my ipod. I have gone back to the store to attempt to repurchase these songs and it only gives me the option to play, but when i try to play them it again says that this computer is not authorized to play this song. does anyone know how to fix this?