Talk:NPC/@comment-4674838-20111217024904/@comment-4674838-20111221070645

Certain conditions such as nighttime and a goblin invasion prevent NPCs from spawning. If you have enough valid housing, and you've met the other conditions for the NPCs then they will eventually spawn/respawn and move into your house.

So, either you're not meeting the NPC conditions (50 silver for merchant etc.), or there's an NPC non-spawn condition on your world (goblin invasion), or your housing is not suitable. If it's not one of those 3 issues, which I'm almost certain it is, then it's a bug.