My iTunes won't stay authorized PLEASE HELP ME!?

I just bought a song off of iTunes and when I went to play it, it said "This computer is not authorized to play this song," blah blah blah. So I authorized it and it accepted it and everything so I clicked PLAY again it the same thing came up! I closed my iTunes and reopened it, and then deleted the song and rebought it even but it's doing the same thing. All my previously purchased songs are working, and I also just downloaded the free song of the week and it let me play that too.

What can I do to fix this problem please I am so frustruated!
I deauthorized my computer and went to the Apple help site and did as it said. Still it didn't work. So I bought another song and THIS ONE DOES PLAY! I am so confused as to why this ONE SONG won't play.