My friend had a demo of Wake Up (by Suicide Silence) and it worked well. He bought the song, and the game no longer recognizes it. There's nothing wrong with it, the game just doesn't add it to the setlist. We have tried everything and nothing works. He has 350+ DLC songs with 50-100 from RBN so it's just random.
We know it's a problem with Rock Band 3, because he can play it in Rock Band 2. However, having to boot up RB2 for one specific song kinda sucks.
I saw somebody earlier having the same problem with that two part awesome song by Rishloo. It's just random because I have that song and it works fine. This is a very tricky bug in RB3 and I have no idea what can cause it.
Anyways, HMX still should try to find out why exactly this happens.