This is the function that I use to submit scores that works for me
- (void) reportScore: (int64_t) reportScore forLeaderboardID: (NSString*) identifier
{
GKScore *scoreReporter = [[GKScore alloc] initWithLeaderboardIdentifier: identifier];
scoreReporter.value = reportScore;
scoreReporter.context = 0;
NSArray *scores = @[scoreReporter];
[GKScore reportScores:scores withCompletionHandler:nil];
}
Also, make sure your leaderboard is either live or added to the new version submission. itunesConnect > Your App > View Details of New Version > Game Center add the new leaderboard if it hasn't been added already.