WPF Grid splitter is not working in my application

I am using below code and gridspliter is not moving to let and right. When trying to move the gridsplitter, it is not moving. Kindly tell me what is wrong in the below shown xaml code ?

    <Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="40"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition MinWidth="100" />
        <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>

    <StackPanel Grid.Row="0" Grid.ColumnSpan="2" Grid.Column="0" Orientation="Horizontal" Margin="5" HorizontalAlignment="Center" VerticalAlignment="Center" Height="30" Width="Auto">
        <TextBlock Text="Your Name " FontFamily="Verdana" Margin="5" HorizontalAlignment="Center" VerticalAlignment="Center" FontSize="12" FontWeight="Bold" />
        <ComboBox Name="cboAlertName" ItemsSource="{Binding stringnames}" Height="25" Width="300"  HorizontalAlignment="Center" VerticalAlignment="Center" Focusable="False"
                  IsTextSearchEnabled="True" StaysOpenOnEdit="True" IsReadOnly="True" IsEditable="True" Text="-- Select your Name --">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <CheckBox Width="100" Height="23" Content="{Binding}" IsChecked="{Binding IsChecked}" />
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </StackPanel>

    <TextBlock Text=" 1st col" Width="100" Height="25" Grid.Row="1" Grid.Column="0" FontSize="16" Margin="10" />

    <GridSplitter HorizontalAlignment="Stretch" ResizeDirection="Columns" ResizeBehavior="PreviousAndNext" Grid.RowSpan="2"              
                  VerticalAlignment="Stretch" Grid.Column="0" Grid.Row="1" Width="5" Background="Silver">
    </GridSplitter>

   
    <TextBlock Text=" 2nt col" Width="100" Height="25" Grid.Row="1" Grid.Column="1" FontSize="16" Margin="10" />
    
</Grid>