The very first time a villager succeeds at "Studying strange plant" for each of the 6 plants, you will see a message "Your villagers now understand this herb." Every time you drop them on a given plant after that first time, they will gain healing skill if they are successful. If that puzzle is not complete, you have most likely missed one of the plants.

Did you check the herb locations map to make sure you haven't overlooked one of the 6 plants?