Why won't this NPC move in?

I'm playing the 3DS version of Terraria, and I just recently beat the Wall of Flesh. Now I have an NPC (named "Shiitake") wanting to settle down, but despite there being plenty of suitable dwellings hasn't moved in.

Is there an NPC that requires some sort of specific furniture to move in?

Solution 1:

"Shiitake" is one of several possible names for the Truffle npc.

His house has an additional condition to be met on top of the usual House requirements: His house must be built in a surface-based Mushroom biome (besides being in Hardmode).

This can be achieved by having a total of 100 pieces of mushroom grass in a single place at the same time.

To make one such biome, place 100 mud blocks in a small area, and cover them all with Mushroom Grass (which you get by planting mushroom grass seeds into the mud, which you get from farming blue mushrooms from their natural biomes). The grass naturally spreads across mud when planted.

For further details, see the wiki article on Truffle