For every day that you complete certain activities with your Pokémon GO friend, you’ll grow your Friendship Level. There are five Friendship Level milestones: Friends, Good Friends, Great Friends, Ultra Friends and Best Friends. There was a bug that prevented increasing the level of your Good Friend status; however, Niantic has confirmed that this particicular issue has been resolved:
Source: Pokémon GO Japan @PokemonGOAppJP
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