If someone doesn't follow you back, it could mean they've either unfollowed you, blocked you, or that they never followed you to begin with.
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If they don't follow you, their Snapscore will be hidden.
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If you mutually follow each other, you'll see their Snapscore, which represents the total sum of all snaps (photos and videos) they've sent and received in the app.
If you're wondering whether someone has unfriended you, tap and hold on their name to see their Snapcode — the yellow ghost icon that looks like this: