Song won't unlock?

[n4sc4r]

I had no idea where to put this, so move it if necessary.

I bought rock band 2 on the launch day, and went through the whole career* that day to unlock all of the songs. (* By career, I mean the Tour Challenges) After that I started doing quickplay. Every time I changed the organization to downloadable content and Rock Band 2, when scrolled over Rock Band 2, it would only show 83 songs.
I checked to see what song was missing today, and it was Shoulder to the Plow. I remembered it from the "Impossible Marathon 2" challenge. I went to check, and sure enough, it shows I have beaten it. Also, "Almost Easy", "Panic Attack", and "Visions", all from that set I believe, are unlocked and show my scores from the challenge in quickplay. Therefore, I figure this must be a glitch.
While it is not that bad, it is slightly annoying, and I don't want to play through Band World Tour until I unlock it if it isn't necessary, so I would like to know if there is a way to unlock it. So, can anybody help me. Do I just need to go through the challenge again or what?

[Phane7]

The only way I managed to unlock everything was by playing through the first marathon challenge in each tier. I didn't have to play through the second one in each tier, just the first.

[ShawnKDesign]

Little did I know.. I had beaten all the impossible challenges thinking the last band impossible challenge was it. There should be a disclaimer that you need to be able to unlock Visions and beat it before proceeding to the Endless Setlist 2... I got all the achievements except for drum specific ones (my drum is broken at the moment), so I went into the Endless Setlist 2 as a Hard guitar. I flew through it then it came to Visions... 20% into the song and I failed miserably. I retried several times and finally decided to quit before I lost more fans... I spent all that time for nothing. I almost cried and messeged my Xbox live buddy, Mike, the expert drummer (and platinum at that) about it. He was kind enough to give me encouraging words..
I know I'm being a whiner.. Sorry. But thanks for the tip. I will try the 1st mararthon challenge in each tier. Good to know.

GT: ShawnKDesign