@noobsauceg7: It's not the greatest list, but this thread shows what songs have been pulled. If you previously purchased any of the pulled songs, they should still be available for Rock Band 4, if I'm not mistaken.
Yes, this has been a problem for almost anyone with a decent amount of DLC songs. The solution is quite simple: wait. Harmonix is uploading over 1000 songs to two consoles. Errors are inevitable, so wait it out and keep an eye on those problem tracks. If it's still a problem sometime after launch (I'd say, give it a week, at least), go bug Harmonix on Twitter.
Just so you know: for whatever reason, entire packs and albums cannot be downloaded. Instead, you'll have to go into the packs/albums and download song-by-song. It's a pain, yes, but Harmonix claims that it's a symptom of the services. (Evidently, Rocksmith went through the same thing with bringing their DLC to the new consoles.)