iTunes Plus "Buy" button missing

I cannot upgrade individual songs using iTunes Plus. The only options I have are to uprade albums or "Buy All". There are over 400 songs listed but my only choice is to upgrade all. However, when I log in using a different computer, I can buy individual songs. Already worked for a week with iTunes Store support and got nowhere. They determined it's not my account. So it seems to be some sort of software conflict. Has anyone else had this problem and been able to resolve it? And upgrading via another machine and downloading via "Purchased" function had a whole different set of problems which make that an unworkable option.