The following xaml does not execute the action specified for the StoryboardCompletedTrigger:

<Grid

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"

xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions">

<Grid.Resources>
    <Storyboard x:Key="anim">
        <ColorAnimation Storyboard.TargetName="rect" Storyboard.TargetProperty="Fill.Color" To="Green"/>
    </Storyboard>
    <ControlTemplate x:Key="controlTemplate" TargetType="Control">
        <StackPanel x:Name="root" Background="Silver">
            <Button Content="Start Anim">
                <i:Interaction.Triggers>
                    <i:EventTrigger EventName="Click">
                        <ei:ControlStoryboardAction Storyboard="{StaticResource anim}"/>
                    </i:EventTrigger>
                    <ei:StoryboardCompletedTrigger Storyboard="{StaticResource anim}">
                        <ei:ChangePropertyAction TargetName="rect" PropertyName="Height" Value="50"/>
                    </ei:StoryboardCompletedTrigger>
                </i:Interaction.Triggers>
            </Button>
            <Rectangle x:Name="rect" Fill="Red" Height="20" Margin="10"/>
        </StackPanel>
    </ControlTemplate>
</Grid.Resources>

<Control Width="200" Height="100" Template="{StaticResource controlTemplate}"/>
</Grid>