"This computer is not authorized to play ."

I clicked on a song I purchased a bit over a year ago ("Sail" by AWOLNATION) and I get a prompt: "This computer is not authorized to play 'Sail.' Would you like to authorize it?"

Well, it IS authorized, as witnessed by the fact that I've been listening to music on it all night. So I 'authorize' it -- again -- and then I get: "This computer is already authorized."

I only have one Apple ID. I have two computers authorized. The other is a Windows machine and it was only recently (as in within the last 30 days) authorized because it did not have iTunes installed prior to that.

I found something about deleting a directory under /users/shared/ called SL Info (I believe that's right), but there IS no folder called that on my system.

I have a MacBook Pro 17" that's 4 years old, running the very latest updates of everything available, including OS X and iTunes.

Anyone got any ideas how I can listen to content I bought and paid for on THIS computer, which is authorized, but claims not to be for this one song? (And maybe others, but I haven't run across any others, yet.)