I have a similar issue. I have 1 macbook pro with 2 user logs, my girlfriend and mine. She has always been able to stay signed into iTunes on her log and do whatever she pleases without her iTunes affecting mine, and on my user log, I've always been able to signed into my iTunes account and iTunes Match without any affect on hers. I have my music and she has hers. Until recently I've been logged out of my iTunes Match account, cannot download purchases and cannot log back into my iTunes Match account. I've spoken to technical support via e-mail and they havent been much help but instead pointing me to this article. http://support.apple.com/kb/HT4627 So in my case 2 user logs with separate iTunes account is no good, Apple pretty much wants everyone to have their own mac to use with iTunes account. This is stupid. Maybe you have a similar situation like mine, where someone else uses a different log and iTunes on that log. It will cause your account to be locked out.