You might already know this, but in case you don't.. it's got to be hardmode, raining, and you have to be in the forest.
Terraria Community Forums
Unit One
RavenChan
I am in hardmode. I don't know if making my forest hallow has anything to do with it, but on my PS3 save, they spawned in a hallowed forest no problem.
Unit One
If you're having spawning issues with it, I'd stick to pure forest in the rain.
RavenChan
Welp, time to finally use the clentaminator I got :\
XoArK
They can spawn in the hallow.