iOS Game Center: Scores not showing on leaderboard in sandbox
Solution 1:
I ran into the same issue with projects I've worked on.
The GC Leaderboards (Sandbox/live) will only return scores if there are two or more players on the Leaderboards.
So make sure you either submit the score with 2 different accounts on one 1 device or 2 different accounts on separate devices.
Solution 2:
Try changing timeScope.
Sometimes GKLeaderboardTimeScopeAllTime & GKLeaderboardTimeScopeToday returns nothing but GKLeaderboardTimeScopeWeek works fine. I think it's a GC sandbox server problem.
Hear us Apple: use some of those billions to make your web services better.