Terraria – Unable to get merchant NPC

During the daytime of 15 minutes, only 1 NPC can spawn every 2 minutes, the NPC must not exist yet and the necessary conditions (separate houses, …) must be met. If multiple NPCs are eligible, then only the first eligible will spawn. It is checked in the following order:

• Guide: No conditions.
• Merchant: House, players have more than 50 silver together (in their inventory).
• Nurse: House, used a crystal heart.
• Arms Dealer: House, carry a gun in your inventory.
• Dryad: House, must have killed a boss.
• Demolitionist: House, Merchant, explosive material in inventory.
• Clothier: House, Skeletron killed.

Exceptions to the single NPC rule, which spawn before the order given above:

• Second Merchant: House, at least 3 players having more than 10 gold together.
• Second Nurse: House, at least 3 players having 20 hearts (i.e., 400 combined health or more).
• Third Merchant: House, at least 5 players having more than 5 platinum together.

There is no order in which they spawn over time. But they can't spawn together, thus first in order…

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.