PerformLayout after adding new nodes

Below is a slimmed down version but has the relevant parts in a generic language. I create a diagram by calling RenderGraph and it works fine. If I call the method again it adds the nodes however it does not perform a new layout. The nodes are under or overlapping the current nodes. IS there something missing like invalidating existing or what not?

Func RenderGraph( DataTableofParentChild as Datatable)

    GoViewer.Dock = DockStyle.Fill
    Digraphlayout.Document = GoViewer.Document
     Loop Datatable DataTableofParentChild
                    nObjectNode =  New GoBasicNode
                    nDependentNode  = New GoBasicNode
                   AddLink(nObjectNode, nDependentNode)
    end Loop

end Func