The grid width didn't update with data binding

<DataTemplate x:Key="SceneryInTimelineItemTemplate">
    <Grid HorizontalAlignment="Left" Width="{Binding ItemWidth}" MinWidth="50" Height="140" Background="Black">
        <TextBlock HorizontalAlignment="Right" Text="{Binding Hours}"/>
        Something else...

    </Grid>
</DataTemplate>

I have binding data to Grid width, it works when the first time to show, but when i update the width data, it didn't change anything:

    current.ItemWidth = 500; // the grid width is not changed for display
    current.Hours= 50;       // the txt is updated

I can't understand, could anyone help?