does iTunes DRM depend on the wifi connection with which the purchase was made
Blatantly stolen from Apple's support website, Make sure iTunes is closed and go into your iTunes folder and delete the following files:

iTunes Library Extras.itdb
iTunes Library Genius.itdb

You may even have an additional "Extras" file with a time/date stamp on it and delete that as well.

Of course, please do this at your own risk of losing any preferences that these files may contain. I don't know exactly what these files do, but I've done this fix for a number of iTunes related issues and it works almost every time.

Hope this helps!

Apple Support Community