Terraria – Why won’t the lucky coin drop after 69 pirate invasions

There's three reasons why an NPC might not respawn:

1. They don't have appropriate housing.
2. Their spawn conditions aren't met anymore.
3. There's a goblin invasion going on.

Did a clown blow up parts of your base and make it uninhabitable to NPCs?

If not, there might be an invasion going on that you didn't notice, if you built your base away from the original spawn point (which is the very middle of the map). Go there, and start killing goblins until it says that they've been defeated.

Also, certain NPCs need their initial spawn conditions to respawn. For example, to get your Demolitionist back, you'll need to carry an explosive around with you until he comes back.

All you have to do is make a volcano with lava in the middle. Check this vid out: