As someone with way too many songs in their library, I admit that I probably couldn't list every single one from memory alone. Last year I had to manually re-download everything to a new Xbox HDD, I ended up temporarily losing Laid To Rest (sadface).
 
I did do a search on the Google with no luck, but does anybody know why some DLC is absent from my RB3 library? i admit, this is as far as I've looked. Again; lots of songs: But I have noticed Rock Your Socks (Ten. D) and The Wagon (Dino. Jr) to be missing - It's worth noting that these tracks are also not appearing on the RockBand.com leaderboards. I've tried re-downloading, and it says it's there on my HDD, it just lists it in the Music Store as Purchased rather than In Library.
 
Help :(
Thanks
 
EDIT: And a thought, it would be nice if the Music Store had a dedicated section for music you've purchased. Really nice.
 
EDIT 2: 666, & Metal on Metal (Anvil) also appear to be suffering the same problem :(

I really hope that none of my DLC is missing, because honestly I may never know. I have so much. Not sure what could be causing missing songs. Did you try deleting the songs from the dashboard and redownloading?

[John] Drake actually just responded to this question earlier today, and I added it to the official export announcement for easy access. Here’s the link:

http://www.rockband.com/forums/showthread.php?204348-Rock-Band-2-to-Rock-Band-3-Export-Info

Basically, we weren’t able to support these songs for RB2 EU disc export which affected their transfer as DLC tracks in the US.

4. People have been mentioning other missing DLC (The Guess Who, Dinosaur Jr., Tenacious D are some examples). Is this a bug/glitch, or will these be permanently missing from Rock Band 3?

We’re working on a fix for these songs. There was an unexpected audio issue with these tracks, which are all part of the Week 96 DLC releases. We should have more info soon.

I am glad the second point is just a glitch, I hadn't noticed I was missing Tenacious D yet, but that would have bummed me out eventually. 
 
As for the Rock Band 1 songs, I don't get that answer at all. I read the post, and understand for Heir Kommt Alex and Rock n' Roll Star, but Soundgarden and Chillis don't make any sense, as they were main set-list, on disc tracks worldwide. I liked those 2 songs, so it's a shame. It would be nice to see a proper explanation.

" there's a rumour going around that there'll be a Guitar Hero: Chili Peppers. "

"Still, I find it kind of weird how they have to re-license those songs again... one re-license for an export onto the hard drive makes sense, but they should just be able to treat them like DLC at that point, which wouldn't require re-licensing. "

"  As for the Rock Band 1 songs, I don't get that answer at all. I read the post, and understand for Heir Kommt Alex and Rock n' Roll Star, but Soundgarden and Chillis don't make any sense, as they were main set-list, on disc tracks worldwide. I liked those 2 songs, so it's a shame. It would be nice to see a proper explanation. "