Nomads, Merchants, and Necromancers spawn in the four corners of your village, about 120 blocks from the town hall.

Each day the Merchant will choose a corner. If he cannot spawn in the corner, you will receive a message in chat saying "Merchant has failed to find a way to village."

Nomads will do the same once they have determined if they spawn. They may choose a different corner to spawn than the Merchant.

If the Necromancer is able to spawn, he follows the same pattern. You will receive a chat message that he has spawned.

If the Visitors cannot locate a path to the Town Hall from the corner, they will also not spawn. This could be caused by an excess of water (such as a swamp or river) cutting off the pathing system, or 2 high blocks preventing the visitors from climbing up to the Town Hall.

Visitors cannot spawn in water but can spawn on full blocks.

If your corners are spawnable but you are not getting any nomads, check your render distance. If it is less than 12 the computer may not be rendering the corner where they spawn. If the corner is not rendered they cannot walk into the village.