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It might be because you generate tokens that are not associated to a referer.
Try by setting IdentityManager.Current.TokenGenerationReferer = <substring of your SL app URL> (e.g "http".
Without the code the only thing that we can do is to guess what you might possibly be doing wrong.
Post the code.
 private void login_Click(object sender, RoutedEventArgs e)
        {
            string url = "https://www.arcgis.com/sharing/generateToken";
            IdentityManager.Current.TokenGenerationReferer = <My IP Address which i got form http://Whatsmyip.com>;
            IdentityManager.Current.GenerateCredentialAsync(url, UserName.Text, Password.Password,
                (credential, ex) =>
                {
                    if (ex == null)
                    {
                       
                        AddFeatureLayers(credential.Token);
                    }
                });
        }
public void AddFeatureLayers(credential.Token){
 MyFeatureLayer = new FeatureLayer
            {
                ID = "TrucksLayer",
                Url = "<AGOL Feature Layer URL>",
                Token = Token              
            };
            MyFeatureLayer .OutFields.Add("*");
            MyFeatureLayer .Initialized += new EventHandler<EventArgs>(MyFeatureLayer_Initialized);
            MyFeatureLayer .InitializationFailed += new EventHandler<EventArgs>(MyFeatureLayer_InitializationFailed);
            _map.Layers.Add(MyFeatureLayer );
}
 void MyFeatureLayer_Initialized(object sender, EventArgs e)
        {
            Console.WriteLine("Layer  initialized");
        }
        void MyFeatureLayer_InitializationFailed(object sender, EventArgs e)
        {
            Console.WriteLine("Layer not initialized");
        }
					
				
			
			
				
			
			
				
			
			
			
			
			
			
		
 public static Task<string> GetToken(string userName, string password)
        {
            var tcs = new TaskCompletionSource<string>();
            string jsonData = "f=json&username=" + userName + "&password=" + password + "&referer=" + "http://arcgis.com" + "&request=getToken";
            WebClient getTokenClient = new WebClient();
            getTokenClient.Headers["Content-Type"] = "application/x-www-form-urlencoded";
            getTokenClient.UploadStringCompleted += (sender, e) =>
            {
                JObject obj = JObject.Parse(e.Result.ToString());
                tcs.TrySetResult(obj["token"].ToString());
            };
            getTokenClient.UploadStringAsync(new Uri("https://www.arcgis.com/sharing/rest/generateToken"), "Post", jsonData);
            return tcs.Task;
        }