sdf model is not showing up correct

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Hi, I am not able to inserting my sdf model correctly into Gazebo7. If I start my model as a world file including the ground_plane and the sun it is showing up properly.

<?xml version="1.0"?>
<sdf version="1.5">

<!-- environment -->
  <world name="default">
    <include>
      <uri>model://ground_plane</uri>
    </include>
<!-- light -->
    <include>
      <uri>model://sun</uri>
    </include>
<!-- Model -->
    <model name="vehicle">
      .......
     </model>
  </world>
</sdf>

As soon I am trying to start it as a model it will be displayed as boxes. Only changes is the following code:

<?xml version="1.0"?>
<sdf version="1.5">

  <model name="vehicle">
      ......
  </model>
</sdf>

The MODEL_PATH is set correctly, it also displays the correct name of the model-file which typed into the .config file. Maybe somebody can help me???