WPF 中的子菜单模板错误

我正在使用标题菜单模板以及子菜单标题模板。主标题菜单运行良好，但子菜单不起作用。它不显示。

我的 Wpf XAML 代码（子菜单标题模板）：

    <ControlTemplate x:Key="{x:Static MenuItem.SubmenuHeaderTemplateKey}"  TargetType="{x:Type MenuItem}">
        <Border x:Name="Border" BorderThickness="1">
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto"    SharedSizeGroup="Icon" />
                    <ColumnDefinition Width="*" />
                    <ColumnDefinition Width="Auto"   SharedSizeGroup="Shortcut" />
                    <ColumnDefinition Width="13" />
                </Grid.ColumnDefinitions>
                <ContentPresenter x:Name="Icon"  Margin="6,0,6,0" VerticalAlignment="Center"  ContentSource="Icon" />
                <ContentPresenter x:Name="HeaderHost" Grid.Column="1" ContentSource="Header"  RecognizesAccessKey="True" />
                <TextBlock x:Name="InputGestureText"  Grid.Column="2" Text="{TemplateBinding InputGestureText}" Margin="5,2,2,2" DockPanel.Dock="Right" />
                <Path Grid.Column="3" HorizontalAlignment="Center" VerticalAlignment="Center" Data="M 0 0 L 0 7 L 4 3.5 Z">
                    <Path.Fill>
                        <SolidColorBrush Color="White"  />
                    </Path.Fill>
                </Path>
                <Popup x:Name="Popup" Placement="Right" HorizontalOffset="-4" IsOpen="{TemplateBinding IsSubmenuOpen}" AllowsTransparency="True" Focusable="False" PopupAnimation="Fade">
                    <Border x:Name="SubmenuBorder" SnapsToDevicePixels="True" Background="RoyalBlue"  BorderThickness="1">
                        <Border.BorderBrush>
                            <SolidColorBrush Color="RoyalBlue" />
                        </Border.BorderBrush>
                    </Border>
                </Popup>
            </Grid>
        </Border>
        <ControlTemplate.Triggers>
            <Trigger Property="Icon" Value="{x:Null}">
                <Setter TargetName="Icon"   Property="Visibility"  Value="Collapsed" />
            </Trigger>
            <Trigger Property="IsHighlighted" Value="true">
                <Setter Property="Background" TargetName="Border">
                    <Setter.Value>
                        <LinearGradientBrush EndPoint="0.5,1"  StartPoint="0.5,0">
                            <GradientStop Color="Transparent" Offset="0" />
                            <GradientStop Color="DeepSkyBlue" Offset="1" />
                        </LinearGradientBrush>
                    </Setter.Value>
                </Setter>
                <Setter Property="BorderBrush" TargetName="Border">
                    <Setter.Value>
                        <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                            <GradientStop Color="DeepSkyBlue"   Offset="0" />
                            <GradientStop Color="Transparent"  Offset="1" />
                        </LinearGradientBrush>
                    </Setter.Value>
                </Setter>
            </Trigger>
            <Trigger SourceName="Popup" Property="AllowsTransparency" Value="True">
                <Setter TargetName="SubmenuBorder" Property="CornerRadius" Value="4" />
                <Setter TargetName="SubmenuBorder" Property="Padding" Value="0,3,0,3" />
            </Trigger>
            <Trigger Property="IsEnabled" Value="false">
                <Setter Property="Foreground">
                    <Setter.Value>
                        <SolidColorBrush Color="DeepPink" />
                    </Setter.Value>
                </Setter>
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

申请设计：

 <Grid>
    <Menu Height="24" VerticalAlignment="Top" Style="{DynamicResource HiMicrosoft}">
        <MenuItem Header="Media" Width="48">
            <MenuItem Header="Open Files" Template="{DynamicResource MI }"/>
            <MenuItem Header="Open Folder" Template="{DynamicResource MI }"/>
            <MenuItem Header="Open Playlist" Template="{DynamicResource MI }"/>
            <MenuItem Header="Open Recent" Template="{DynamicResource MI }"/>
            <Separator Style="{DynamicResource {x:Static MenuItem.SeparatorStyleKey} }"/>
            <MenuItem Header="Hide" Template="{DynamicResource MI }"/>
            <MenuItem Header="Exit" Template="{DynamicResource MI }"/>
        </MenuItem>
        <MenuItem Header="Playback" >
            <MenuItem Header="Play/Pause" Template="{DynamicResource MI }"/>
            <MenuItem Header="Stop" Template="{DynamicResource MI }"/>
            <Separator Style="{DynamicResource {x:Static MenuItem.SeparatorStyleKey} }" />
            <MenuItem Header="Speed" Template="{DynamicResource MI }">
                <MenuItem  Header="Stop"  Template="{DynamicResource {x:Static MenuItem.SubmenuHeaderTemplateKey}}"/>
            </MenuItem>
        </MenuItem>
    </Menu>
</Grid>

这是最终输出屏幕截图：

当我点击速度时，它什么也没显示。我无法识别错误，或者可能是我使用了错误的方法。

任何帮助将不胜感激！

如果您愿意，我可以提供更多详细信息。只需在下面评论。

提前致谢！