iTunes is not accepting my password, but it works in App Stores.

I had hoped that updating the software would help, but the problem still persists. The password works in everything except for automatic downloads and view my account. It even works for Authorize this computer. When I enter the correct password, the pop up box disappears and reappears unchanged. There is no incorrect password/username message when I do this, but if I do put in a wrong one, the message appears.

Any ideas?