Lunatic Cultist won't spawn!

There used to be some problems in regards to the Lunatic Cultist spawning. I suppose they haven't been quite fixed. In general, to prevent any issues you should avoid altering the dungeon entrance. Specifically removing dungeon bricks. It messes with the boss and the cultists spawn point.

Rebuilding the entrance may solve the issue, however since you already killed the cultist you may have to wait 2 or 3 in-game days. If they still don't spawn, try 5-6 days. If there's still no luck I really wouldn't keep trying on that world.