Symbol invisible

[JOhntwi]

Hello
I have a small problem symbol.
On some pc, I see my symbols and on other pc and mac, my symbols disappears.
Someone has a solution?
It's strange.
I develop with Visual Studio 2010, silverlight 4.

        <Grid.Resources>
            <esri:SimpleMarkerSymbol x:Name="RedMarkerSymbol" Color="Red" Size="8" Style="Circle" />
</Grid.Resources>

CODE BEHIND

var graphic = new Graphic
                                          {
                                              Geometry = new MapPoint(y, x),
                                              Symbol = LayoutRoot.Resources["RedMarkerSymbol"] as Symbol
                                          };

Thank you