Невозможно перетащить ползунок

Я пытаюсь создать собственный слайдер, который выглядит так:

Проблема в том, что я не могу тянуть большой палец. Большой палец фактически перемещается, когда я нажимаю на обе стороны ползунка (т.е. кнопки увеличения/уменьшения), но перетаскивание большого пальца не работает.

Вот XAML:

<Window.Resources>
    <Style x:Key = "RepeatButtonTransparent" TargetType = "{x:Type RepeatButton}">
        <Setter Property = "OverridesDefaultStyle" Value = "true"/>
        <Setter Property = "Background" Value = "Transparent"/>
        <Setter Property = "Focusable" Value = "false"/>
        <Setter Property = "IsTabStop" Value = "false"/>
        <Setter Property = "Template">
            <Setter.Value>
                <ControlTemplate TargetType = "{x:Type RepeatButton}">
                    <Rectangle Fill = "{TemplateBinding Background}"
                           Height = "{TemplateBinding Height}"
                           Width = "{TemplateBinding Width}"/>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <ControlTemplate x:Key = "SliderThumb" TargetType = "{x:Type Thumb}">
        <Grid>
            <Canvas MinHeight = "34" Height = "{TemplateBinding Height}">
                <Grid x:Name = "grip">
                    <Grid.RowDefinitions>
                        <RowDefinition Height = "auto"/>
                        <RowDefinition Height = "20"/>
                    </Grid.RowDefinitions>
                    <TextBlock Grid.Row = "0" Text = "{Binding Value, StringFormat=N0,
                    RelativeSource = {RelativeSource AncestorType = {x:Type Slider}}}"
                    FontSize = "14" VerticalAlignment = "Top"/>
                    <Rectangle Grid.Row = "1" Width = "4" RadiusX = "2" RadiusY = "2"
                    Fill = "#92BF30" VerticalAlignment = "Top" Height = "20"/>
                </Grid>
            </Canvas>
        </Grid>
        <ControlTemplate.Triggers>
            <Trigger Property = "IsMouseOver" Value = "true">
            </Trigger>
            <Trigger Property = "IsDragging" Value = "true">
            </Trigger>
            <Trigger Property = "IsEnabled" Value = "false">
            </Trigger>
        </ControlTemplate.Triggers>
    </ControlTemplate>

    <Style TargetType = "Slider">
        <Setter Property = "Focusable" Value = "False"/>
        <Setter Property = "SnapsToDevicePixels" Value = "true" />
        <Setter Property = "OverridesDefaultStyle" Value = "true" />
        <Setter Property = "Height" Value = "35"/>
        <Style.Triggers>
            <Trigger Property = "Orientation" Value = "Horizontal">
                <Setter Property = "Template">
                    <Setter.Value>
                        <ControlTemplate TargetType = "Slider">
                            <Grid>
                                <Grid.RowDefinitions>
                                    <RowDefinition Height = "auto"/>
                                    <RowDefinition Height = "auto" MinHeight = "{TemplateBinding MinHeight}"/>
                                    <RowDefinition Height = "auto"/>
                                </Grid.RowDefinitions>
                                <TickBar x:Name = "TopTick" Fill = "{TemplateBinding Foreground}"
                                     Height = "4" Margin = "0,0,0,2" Placement = "Top" Grid.Row = "0"
                                     Visibility = "Collapsed"/>
                                <TickBar x:Name = "BottomTick" Fill = "{TemplateBinding Foreground}"
                                     Height = "4" Margin = "0,2,0,0" Placement = "Bottom" Grid.Row = "2"
                                     Visibility = "Collapsed"/>
                                <Border x:Name = "TrackBackground" BorderThickness = "0" Background = "Gray"
                                    Height = "4.0" Grid.Row = "1" VerticalAlignment = "Center"
                                    CornerRadius = "2" Opacity = "0.7" Margin = "0 23 0 0">
                                    <Canvas Margin = "-6,-1">
                                        <Rectangle x:Name = "PART_SelectionRange" Fill = "Gray" Height = "4.0"
                                               Visibility = "Hidden"/>
                                    </Canvas>
                                </Border>
                                <Track x:Name = "Track" Grid.Row = "1" Margin = "0 0 0 0">
                                    <Track.DecreaseRepeatButton>
                                        <RepeatButton Style = "{StaticResource RepeatButtonTransparent}"
                                                  Command = "Slider.DecreaseLarge" Height = "10" Margin = "0 26 0 0"/>
                                    </Track.DecreaseRepeatButton>
                                    <Track.Thumb>
                                        <Thumb x:Name = "Thumb" Margin = "0 0 20 0" Template = "{StaticResource SliderThumb}"></Thumb>
                                    </Track.Thumb>
                                    <Track.IncreaseRepeatButton>
                                        <RepeatButton Style = "{StaticResource RepeatButtonTransparent}"
                                                  Command = "Slider.IncreaseLarge" Height = "10" Margin = "0 23 0 0"/>
                                    </Track.IncreaseRepeatButton>
                                </Track>
                            </Grid>
                            <ControlTemplate.Triggers>
                                <Trigger Property = "TickPlacement" Value = "Both">
                                    <Setter Property = "Visibility" TargetName = "TopTick" Value = "Visible"/>
                                    <Setter Property = "Visibility" TargetName = "BottomTick" Value = "Visible"/>
                                </Trigger>
                                <Trigger Property = "IsSelectionRangeEnabled" Value = "true">
                                    <Setter Property = "Visibility" TargetName = "PART_SelectionRange" Value = "Visible"/>
                                </Trigger>
                                <Trigger Property = "IsKeyboardFocused" Value = "true">
                                    <Setter Property = "Foreground" TargetName = "Thumb" Value = "Blue"/>
                                </Trigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Trigger>
        </Style.Triggers>
    </Style>
</Window.Resources>

<Slider VerticalAlignment = "Center" HorizontalAlignment = "Center" Width = "300"
                Minimum = "0" Maximum = "15"/>

Что я здесь делаю неправильно?