Why does my UserControl not show its content?

I have an application that I wrote long ago in SL 3.x. I have converted it all over to SL 5. Everything works EXCEPT one part. I have a WrapPanel whose children I add dynamically at runtime. I can see in the debugger that everything appears to be okay, no null objects, no complaints in the output window, but I see no content of my WrapPanel child. I can tell that the child was added because the control grows, but nothing shows.

Here is my UserControl:

<UserControl     x:Class="EDBSilverlight.CommonParts.LegendItem"     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"   Background="White"   >   <Grid x:Name="LayoutRoot" Margin="4,2,4,2" >      <TextBlock Text="test" Foreground="Black" FontSize="24" />   </Grid></UserControl>

Here is my code to create instances at runtime:

      procedure CreateLegend;         begin         var legendItem            := new LegendItem;         SaveRatmChart.MyLegendPanel.Children.Add( legendItem );         end;

The LegendItem never shows.  I added a button to create new ones and I can see that the MyLegendPanel which is just a WrapPanel is growing in size and seems to show the content I add.  It is like the LegendItem doesn't paint itself for some reason when it first displays that is beyond me.

Here is that LegendPanel:

<my3:WrapPanel   
   x:Name="MyLegendPanel"   Grid.Row="1"   HorizontalAlignment="Center"   VerticalAlignment="Bottom"   Margin="50,0,50,0"   />

This code worked in SL 3.x.