JUNG: placing tree nodes in order

When adding nodes to my DelegateTree, they don't appear visually in the order I added them.
I've been looking for a solution, but haven't found anything yet.
Is there anyone who might know how to change this?

Thanks in advance!

EDIT: My code

/**
  * Generate a visualization of the decision tree.
  * 
  * @param tree
  *            The decision tree
  * @return A component to be placed inside a JPanel
  */
 public static GraphZoomScrollPane generateTree(Tree tree,
         GraphicalUserInterface gui) {
 
     /* Create a new tree */
     DelegateTree<Node, Edge> graphTree = new DelegateTree<Node, Edge>();
 
     /* Add all nodes and vertices to the tree */
     graphTree.addVertex(tree.getRoot());
     addChildren(tree.getRoot(), graphTree);
 
     /* Create the visualization */
     DynamicTreeLayout<Node, Edge> treeLayout = new DynamicTreeLayout<Node, Edge>(graphTree, 100, 100);
     VisualizationViewer<Node, Edge> vv = new VisualizationViewer<Node, Edge>(
             treeLayout);
     vv.setDoubleBuffered(true);
     vv.setBackground(Color.WHITE);
     vv.getRenderContext().setEdgeLabelTransformer(
             new ToStringLabeller<Edge>());
     vv.getRenderContext().setEdgeShapeTransformer(
             new EdgeShape.Line<Node, Edge>());
     vv.getRenderContext().setVertexLabelTransformer(
             new ToStringLabeller<Node>());
     vv.getRenderer().getVertexLabelRenderer()
             .setPosition(Renderer.VertexLabel.Position.S);
 
     vv.addGraphMouseListener(new ClickNode(gui, vv));
 
     final DefaultModalGraphMouse<Node, Edge> graphMouse = new DefaultModalGraphMouse<Node, Edge>();
 
     graphMouse.setMode(ModalGraphMouse.Mode.TRANSFORMING);
 
     vv.setGraphMouse(graphMouse);
 
     final GraphZoomScrollPane gzsp = new GraphZoomScrollPane(vv);
 
     return gzsp;
 }
 
 /**
  * Recursively add all nodes and edges to the tree.
  * 
  * @param node
  *            The parent node
  * @param tree
  *            The tree visualization
  */
 private static void addChildren(Node node, DelegateTree<Node, Edge> tree) {
     for (int i = 0; i < node.getChildren().size(); i++) {
         tree.addChild(new Edge(node.getChildren().get(i).getEdgeLabel()), node, node
         .getChildren().get(i));
         addChildren(node.getChildren().get(i), tree);
     }
 }
 

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