Thanks.
There is a guide on the description of the game that tells you how to unlock the NPCs. The ones from the shops are picked at random, so sometimes it does take a dumb amount of tries. I am still deciding on how to mitigate this.
One thing I have in mind is to make it so eventually as the game progresses you can interact with all (or at least most) NPCs without having to rely on the shops.
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Gotcha. For other NPCs that I have already unlocked they would appear multiple times, so I guess it's more likely that I simply didn't unlock the character I was looking for.
I think since there are a lot of NPCs in your game, as long as they don't play a major role in pushing the plot, there isn't much need to unlock them in order. Currently what you have is good enough, as in, for players they can unlock a certain number of new NPCs each chapter. However, perhaps you can let the player decide in what order they want to unlock new NPCs (like, New chapter --> run delivery errands for shops --> choose from a list of new NPCs).
I also think that, for repeated shop errands where MC delivers for previously met NPCs, there should be ways to make it more doable or fun. Like, directly switch player to NPC's location; add random encounter events in the middle with flavor texts ( show how NPCs interact with each other perhaps?) or even CGs, if possible.