Why is the model "head" kept falling down through the another model "chassis"?

edit

the code is here:

  <include>
  <uri>model://head</uri>
  <pose>0.090 0 0.16 0 0 0</pose>
  <name>head</name>
  </include>
  <joint name="head_joint" type="fixed">
  <child>head</child>
  <parent>chassis</parent>
  </joint>
  </model>

my chassis code is:

        <link name='chassis'>
        <pose>0 0 .1 0 0 0</pose>

        <collision name='collision'>
          <geometry>
            <box>
              <size>0.2159 0.1524 0.10795</size>
            </box>
          </geometry>
        </collision>

        <visual name='visual'>
          <geometry>
            <box>
              <size>0.2159 0.1524 0.10795</size>
            </box>
          </geometry>
        </visual>
      </link>

I don't get how it just go through the model. Did I used the wrong one or what? I linked head as a child with chassis as parent.

I wanted to attach head on the chassis firmly without letting it go through the chassis. Do you have any idea what to put?

I'm using Gazebo11