ITunes says computer not authorized. When I try to authorize computer it says it is already authorized but will not play song

Question

iTunes says computer not authorized. When I try to authorize computer it says it is already authorized but will not play song. Note. Library was moved from PC to MAC. I am signed in to Itunes with the Apple ID created on the old computer which is different than the Apple ID used to set up the MAC.

Answer 1

I'm experiencing the same thing under iTunes 11.1.1:
iTunes says the computer is not authorized, prompts me to authorize
I enter my credentials, iTunes says computer is already authorized
When I attempt to play song, iTunes starts at bullet point 1 above over again.
My attempted fixes:
I tried everything in this support document, to no avail: http://support.apple.com/kb/TS1389 (Note: I cannot find a SC Info folder on my Mac)
Deauthorize and then reauthorize computer
Delete song and try to redownload from iTunes store - there's only a button that says 'purchased' with no option to redownload it
Note: I am using only one iTunes account, and I double checked the song info - I'm authorizing the correct account - 12 songs from the same album purchased at the same time play fine. As does the song in question - from the finder. Only iTunes stubbornly refuses to play it.
The most odd thing is that iTunes music is DRM free - the song in question was purchased only a few months ago. Thus there's no need to authorize the computer to play it.
I was hoping that iTunes 11.1.1 would cure it - no dice.