Terraria – Highest DPS Melee Weapon Other than Zenith

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.

You can just hook all the statues up to the same timer. They will all activate at the same time. Putting them on different timer's doesn't provide any further benefits.