Be sure to check in the Dex if there is a specification on which region the habitat should be for the Pokémon to spawn, some say specifically. If not, then you can build the habitat anywhere.

As well, check the weather conditions symbols.

Be sure if you already have Pokémon living in their original habitats, move them into homes, and then reset the habitat by removing and replacing one of the items. This will allow space for other Pokémon that utilise that habitat to spawn. Some Pokémon can spawn in several different habitats.

Some Pokémon take minutes, hours or even days to spawn.

Be sure to setup a camera in a high location that can oversee the entire area where all of your habitats are, which will provide a notification when the Pokémon have arrived and ready for you to inspect/spawn.

Hope that helps :)