I'm trying to implement an AngularJS plugin into my application, but I can't really get it to work properly. I might be doing something wrong.

I have two panels that are sortable. I can swap their positions only once. After that they won't swap places anymore. 

Here is an example of my problem:
<http://plnkr.co/edit/44fN6h6VyIft7j7bvs4j?p=preview>

Anyone any idea what the problem is and how I can fix this issue?

---

Plnkr code (complete code in example):
--------------------------------------

```
<button type="button" ng-click="addItem('item1.html');">Add item 1</button>
<button type="button" ng-click="addItem('item2.html');">Add item 2</button>

```

---

```
<div ui-tree class="dd">
    <ol ui-tree-nodes ng-model="list.items" class="dd-list">

        <li ui-tree-node class="dd-item dd3-item" ng-repeat="element in list.items track by $index">
            <div class="panel panel-default">
                <div class="panel-heading clearfix">
                  Heading
                </div>
                <div id="settings-body{{$index}}" class="panel-body" data-nodrag>
                    <ng-include src="element.name"></ng-include>
                </div>
            </div>
        </li>

    </ol>
</div>

```

App.js

```
var app = angular.module('plunker', ['ui.tree']);

app.controller('MainCtrl', function($scope) {
  $scope.list = {};
  $scope.list.items = [];

  $scope.addItem = function(name) {
    $scope.list.items.push({name: name});
  }
});

```