Google Play Services Working, but not Working?

I've been watching and reading tutorials for days, and I'm stumped. Hopefully someone here can shed some light. I integrated the Google Play Services package and modified my script. I have achievements and leaderboards in place, and a debug gui that tells me whether or not user authentication was successful. Just to be safe, I added a button that forces authentication again when pressed. According to debug, authentication works fine. But... There is no sign in window, there is no notification when an achievement requirement is met, and the achievement and leaderboard buttons are unresponsive. Here's the relevant bits of my code:

         using UnityEngine;
         using System.Collections;
         using GooglePlayGames;
         using UnityEngine.SocialPlatforms;
         
         public class GooglePlay : MonoBehaviour {
         
                 void Start ()
             {
                 AuthenticateUser ();
             }
             bool showAuthFailure;
             bool showAuthSuccess;
             void AuthenticateUser()
             {
                 PlayGamesPlatform.Activate ();
                 Social.localUser.Authenticate((bool success) => {
                     if(!success)
                         showAuthFailure = true;
                     if(success)
                         showAuthSuccess = true;
                 });
             }
             public void ShowAchievements()
             {
                 Social.ShowAchievementsUI();
             }
             public void ShowLeaderboard()
             {
                 Social.ShowLeaderboardUI();
             }
          public void PostToLeaderboard(int score)
         {
                 Social.ReportScore (score, "CgkI_7vourIJEAIQHQ", (bool success) => {
                     //TODO handle success or failure
                 });
         }
     public void UnlockAchievement()
         {
                 Social.ReportProgress("Cgkl_7vourlJEAIQAg", 100.0f, (bool success) => {
                     //TODO handle success or failure
                 });
             }
             bool showGUI = true;
             bool showButton = true;
             void OnGUI()
             {
                 GUILayout.BeginArea(new Rect(5, 5, Screen.width/2, Screen.height));
                 if (showButton)
                 {
                     if(GUILayout.Button ("Authenticate!"))
                     {
                         AuthenticateUser();
                         showButton = false;
                     }
                 }
                 if (showGUI)
                 {
                     GUILayout.BeginVertical ("box");
                     if (showAuthSuccess) {
                         if (GUILayout.Button ("Google User Authenticated!"))
                         {
                             showAuthSuccess = false;
                             showGUI = false;
                         }
                     }
                     if (showAuthFailure) {
                         if (GUILayout.Button ("Use Authentication Failed!"))
                         {
                             showAuthFailure = false;
                             showGUI = false;
                         }
                     }
                     GUILayout.Label(Social.localUser.userName);
                     GUILayout.Label(Social.localUser.id);
                     GUILayout.Label(Social.localUser.image);
         
                     GUILayout.EndVertical ();
                 }
                 GUILayout.EndArea ();
             }

When you start the game, the gui clearly displays: Google User Authenticated. Something must be working, right?