Google Play Services leaderboard not updating after high score is beaten?

Hi, I've been developing a game for android and I have wanted to incorporate a leader board so that players can compete with each other for the top score. I have an internal test on the Play Store setup so that friends and I can test everything out to see if it's working. So far I was able to get my score on the leader-board and when people beat my score their score is then put on the top, etc. However once me and someone else both have an existing score on the leader-board it appears that the score's will not update if beaten. It's as if because I already have a score on the leader-board it's not letting me add the higher one.

I have the usual setup for using a leader-board, a separate class to handle authenticating with Google Play Services and using the leader-board, the function in this class that adds the score looks like this:

 public void AddScoreToLeaderboard(string leaderboardID, int score)
     {
         Social.ReportScore((long)score, leaderboardID, success => { });
     }

Then at the end of each game, when the game over menu appears, this line is called which calls the above function in the Leaderboard class (I've blocked out the leader-board ID):

 leaderboard.AddScoreToLeaderboard("exampleID1234", runtime.GetComponent<ScoreTracker>().score);

I've done this since the actual high score is stored in Player Prefs which could be edited by knowledgeable players, so instead at the end of each run the current score stored in my ScoreTracker script is submitted. At this point a player can use one revive though which just means at the end of that run and the game over screen appears again the line above is called once again, which should just update the score if higher regardless.

Is this a bad way about going using the google play leader-board system and is there something I'm doing wrong?

Any help would be greatly appreciated, thanks.