XAML StoryBoard StackPanel

Question

Hi, I am trying to add a storyBoard to my stack panel and bind the trigger to a code behind property, but the stackpanel is not responding to the property changes. here is the code:

<Window x:Class="DirectAccess.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:DirectAccess"
        Title="MainWindow" Height="520" Width="250"
        x:Name="Main">    
    <StackPanel Orientation="Horizontal" >
        <StackPanel.Style>
            <Style>
                <Style.Triggers>
                    <Trigger Property="local:MainWindow.IsLogged"  Value="false">
                        <Trigger.EnterActions>
                            <BeginStoryboard>
                                <Storyboard>
                                    <DoubleAnimation                                   
                                    Storyboard.TargetProperty="Width"
                                    From="250"
                                    To="800"
                                    Duration="0:0:1"/>
                                </Storyboard>
                            </BeginStoryboard>
                        </Trigger.EnterActions>
                    </Trigger>
                </Style.Triggers>
            </Style>
        </StackPanel.Style>
</StackPanel>
</Window>

and the code behind property :

public static readonly DependencyProperty IsLoggedProperty = DependencyProperty.Register("IsLogged", typeof(bool), typeof(MainWindow));
    public bool IsLogged
    {
        get { return (bool)GetValue(IsLoggedProperty); }
        set { SetValue(IsLoggedProperty, value); }
    }

where is the problem...? In fact the storyboard is only performing on application load, but when i set IsLogged property to 'true' on application load, and set it to false afterwards, the application doesn't respond.

thank you in advance.