Ok so all the requirements were right. I was in an artificial crimson that had worked the first time I summoned the brain, the the second time I summoned it, the it said that the brain of cthulhu has awoken and it played that boss roar sound, but no brain. The third time it worked, the fourth time it didn't. Every other time I used the bloody spine the brain would not summon.
Terraria – Brain of Cthulu won’t spawn every other time I use the bloody spine
terraria
Best Answer
If you're getting inconsistent results summoning the Brain, I'd recommend making sure your artificial Crimson biome is large enough to allow it to spawn off-screen and travel to you without being outside the Crimson.