Windows Phone 8.1组合框点击事件don'；不起作用

你能解释一下为什么我的组合框中的Tapped事件没有引发指定的方法吗？

<ComboBox HorizontalAlignment="Center" Margin="19,9.5,19,9.5" Header="Choose the type of the plot" FontSize="15">
                <ComboBoxItem Content="Signal Winner" Tapped="Winner_Signal_Tapped" />
                <ComboBoxItem Content="Filtered signal Winner" Tapped="Winner_Prediction_Tapped" />
                <ComboBoxItem Content="Quality Winner" Tapped="Winner_Quality_Tapped" />
                <ComboBoxItem Content="Error Winner" Tapped="Winner_Error_Tapped" />
                <ComboBoxItem Content="Signal LMS" Tapped="LMS_Signal_Tapped" />
                <ComboBoxItem Content="Filtered signal LMS" Tapped="LMS_Prediction_Tapped" />
                <ComboBoxItem Content="Quality LMS" Tapped="LMS_Quality_Tapped" />
                <ComboBoxItem Content="Error LMS" Tapped="LMS_Error_Tapped" />
            </ComboBox>
private void plotButton_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            if (LMSPlot != null)
            {
                graphGrid.Visibility = Windows.UI.Xaml.Visibility.Visible;
                var line1 = plot1.Series[0] as LineSeries;
                line1.ItemsSource = LMSPlot;
            }
            if (WinnerPlot != null)
            {
                graphGrid.Visibility = Windows.UI.Xaml.Visibility.Visible;
                var line1 = plot1.Series[0] as LineSeries;
                line1.ItemsSource = WinnerPlot;
            }
        }
        catch(Exception ex)
        {
             //for debug purpose
        }
    }

附言：我指出，如果我在ComboBox中只保留4个项目，Tapped事件就可以工作并调用指定的方法。但如果我把5-8，什么都不管用。为什么？

 <ComboBox HorizontalAlignment="Center" Margin="19,9.5,19,9.5" SelectionChanged="ComboBox_SelectionChanged" Header="Choose the type of the plot" FontSize="15">
                <ComboBoxItem Content="Signal Winner" IsSelected="True" Tag="SW"/>
                <ComboBoxItem Content="Filtered signal Winner" Tag="FSW"/>
                <ComboBoxItem Content="Quality Winner" Tag="QW"/>
                <ComboBoxItem Content="Error Winner"  Tag="EW"/>
                <ComboBoxItem Content="Signal LMS"  Tag="SL"/>
                <ComboBoxItem Content="Filtered signal LMS" Tag="FSL" />
                <ComboBoxItem Content="Quality LMS"  Tag="QL"/>
                <ComboBoxItem Content="Error LMS"  Tag="EL"/>
            </ComboBox>

    private void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        ComboBox cb = sender as ComboBox;
        ComboBoxItem selectedItem = cb.SelectedItem as ComboBoxItem;
        switch (selectedItem.Tag.ToString())
        {
            case "SW":
                //first item selected...
                break;
            case "FSW":
                // Second item selected..
                break;
            case "QW":
                break;
        }
    }