public class PrimitiveShape : RadDiagramShapeBase
{
    public PrimitiveShape() : base()
    {
        this.DefaultStyleKey = typeof(PrimitiveShape);
         
    }
}
<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
    xmlns:local="clr-namespace:GSAB.RSProduction.Modules.OfficeTools.ViewModel.Workflow">
 
    <Style TargetType="local:PrimitiveShape">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="local:PrimitiveShape">
                    <Border Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}">
                         
                        <telerik:RadDiagramShape Geometry="{telerik:CommonShape ShapeType=OctagonShape}" />
                     
                    </Border>
 
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>
I'm trying to implement a simple primitive shape by inheriting from RadDiagramShapeBase, but when adding it to the diagram no connectors is showing.

Any suggestions on what I'm doing wrong here would be appreciated.

 Could you please give us more information on what you are trying to achieve? Are you inheriting the base RadDiagramShape class to extend its funcionality or to have a different look and feel? If it is the latter, then you can just change the default template or content template, without subclassing it. Moreover, in the inherited shape's template you should not be using the original shape. I would recommend using the default shape style as a base for your custom one. The default styles we are shipping in the installation directory. Its full path should look something like this : C:\Program Files (x86)\Telerik\RadControls for Silverlight Q1 2012\Themes\Metro\Themes\Metro\DiagramShapes.xaml