Triggers not working with Text-Box in Data-Grid's Template (Silverlight-MVVM)

<UserControl xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk"  x:Class="MVVMEventTriggerDemo.Views.MainView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:viewModel="clr-namespace:MVVMEventTriggerDemo.ViewModels"
    xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
    xmlns:si="clr-namespace:Expression.Samples.Interactivity;assembly=Expression.Samples.Interactivity"
    Height="600" Width="700">
    <UserControl.Resources>
        <viewModel:MainViewModel x:Key="MainViewmodel"/>
    </UserControl.Resources>

    <Grid x:Name="LayoutRoot" removed="White" DataContext="{StaticResource MainViewmodel}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="258*" />
            <ColumnDefinition Width="262*" />
        </Grid.ColumnDefinitions>
        <TextBox x:Name="t1" Text="{Binding EmployeeName, Mode=TwoWay}" Width="100" Height="30" Margin="142,77,20,289" Grid.Column="1">
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="TextChanged">
                    <si:CallDataMethod Method="UpdateText"/>
                    <si:ShowMessageBox Caption="Thank you"
                                       Message="Thanks for trying the Example"
                                       MessageBoxButton="OK"/>
                    <si:SetProperty TargetName="t1" PropertyName="Background" Value="PaleGoldenrod"/>
                </i:EventTrigger>
            </i:Interaction.Triggers>
        </TextBox>

<-- Till here working is ok of textBox with UpdateText- Command.-->
<--But in dataGrid's textBox No Command gets fired -->
 <sdk:DataGrid 
            x:Name="dgDevice" 
            AutoGenerateColumns="False" 
            Margin="13,13,13,13"
            BorderThickness="1,0,1,1"
            RowBackground="White"
            VerticalScrollBarVisibility="Auto" Width="320" 
            Opacity="0.9"
            removed="White"
            GridLinesVisibility="None"
            HeadersVisibility="None"
            HorizontalScrollBarVisibility="Disabled"
            ItemsSource="{Binding ActualColors,Mode=TwoWay}">
            <sdk:DataGrid.Columns>
                <sdk:DataGridTemplateColumn Width="320" >
                    <sdk:DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <Grid Width="auto">
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="30*"  />                                  
                                </Grid.ColumnDefinitions>
                               
                              <TextBox x:Name="t21" Text="{Binding Start, Mode=TwoWay}" Width="100" Height="30" Margin="142,77,20,289" Grid.Column="1">
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="TextChanged">
                    <si:CallDataMethod Method="UpdateText"/>
                    <si:ShowMessageBox Caption="Thank you"
                                       Message="Thanks for trying the Example"
                                       MessageBoxButton="OK"/>
                    <si:SetProperty TargetName="t21" PropertyName="Background" Value="PaleGoldenrod"/>
                </i:EventTrigger>
            </i:Interaction.Triggers>
        </TextBox>

                            </Grid>
                        </DataTemplate>
                    </sdk:DataGridTemplateColumn.CellTemplate>
                </sdk:DataGridTemplateColumn>
            </sdk:DataGrid.Columns>
        </sdk:DataGrid>
</Grid>
</UserControl>