imported models couldn't collide each other

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Have a nice day! azeey.

Unfortunately, it's weird in my testing. I wrote the sdf file based on this example code /usr/share/ignition/ignition-gazebo6/worlds/heightmap.sdf which has ground plane as below code. this example code has already ground plane and if I just add SUVs, it shows that it is getting worse result. SUVs are through the ground plane without any collision. To avoid this, I had to use ground area 'https://fuel.ignitionrobotics.org/1.0/OpenRobotics/models/Ground Plane'.

[solved] That's a plugin reason. by changing loading part of physics plugin from 'heightmap.sdf' to 'tunnel.sdf', it works fine.

<physics name="1ms" type="ignored">
  <max_step_size>0.001</max_step_size>
  <real_time_factor>1.0</real_time_factor>
</physics>

[my testing code]

<sdf version="1.6"> <world name="heightmap"> <physics name="1ms" type="ignored"> <dart> <collision_detector>bullet</collision_detector> </dart> </physics>

<light type="directional" name="sun">
  <cast_shadows>true</cast_shadows>
  <pose>0 0 10 0 0 0</pose>
  <diffuse>0.8 0.8 0.8 1</diffuse>
  <specular>0.2 0.2 0.2 1</specular>
  <attenuation>
    <range>1000</range>
    <constant>0.9</constant>
    <linear>0.01</linear>
    <quadratic>0.001</quadratic>
  </attenuation>
  <direction>-0.5 0.1 -0.9</direction>
</light>

<model name="sphere_red">
  <pose>30 30 10 0 0 0</pose>
  <link name="sphere_link">
    <inertial>
      <inertia>
        <ixx>3</ixx>
        <ixy>0</ixy>
        <ixz>0</ixz>
        <iyy>3</iyy>
        <iyz>0</iyz>
        <izz>3</izz>
      </inertia>
      <mass>3.0</mass>
    </inertial>
    <collision name="sphere_collision">
      <geometry>
        <sphere>
          <radius>0.5</radius>
        </sphere>
      </geometry>
    </collision>
    <visual name="sphere_visual">
      <geometry>
        <sphere>
          <radius>0.5</radius>
        </sphere>
      </geometry>
      <material>
        <ambient>1 0 0 1</ambient>
        <diffuse>1 0 0 1</diffuse>
        <specular>1 0 0 1</specular>
      </material>
    </visual>
  </link>
</model>

<model name="sphere_green">
  <pose>30 -30 10 0 0 0</pose>
  <link name="sphere_link">
    <inertial>
      <inertia>
        <ixx>3</ixx>
        <ixy>0</ixy>
        <ixz>0</ixz>
        <iyy>3</iyy>
        <iyz>0</iyz>
        <izz>3</izz>
      </inertia>
      <mass>3.0</mass>
    </inertial>
    <collision name="sphere_collision">
      <geometry>
        <sphere>
          <radius>0.5</radius>
        </sphere>
      </geometry>
    </collision>
    <visual name="sphere_visual">
      <geometry>
        <sphere>
          <radius>0.5</radius>
        </sphere>
      </geometry>
      <material>
        <ambient>0 1 0 1</ambient>
        <diffuse>0 1 0 1</diffuse>
        <specular>0 1 0 1</specular>
      </material>
    </visual>
  </link>
</model>

<model name="sphere_blue">
  <pose>-30 30 10 0 0 0</pose>
  <link name="sphere_link">
    <inertial>
      <inertia>
        <ixx>3</ixx>
        <ixy>0</ixy>
        <ixz>0</ixz>
        <iyy>3</iyy>
        <iyz>0</iyz>
        <izz>3</izz>
      </inertia>
      <mass>3.0</mass>
    </inertial>
    <collision name="sphere_collision">
      <geometry>
        <sphere>
          <radius>0.5</radius>
        </sphere>
      </geometry>
    </collision>
    <visual name="sphere_visual">
      <geometry>
        <sphere>
          <radius>0.5</radius>
        </sphere>
      </geometry>
      <material>
        <ambient>0 0 1 1</ambient>
        <diffuse>0 0 1 1</diffuse>
        <specular>0 0 1 1</specular>
      </material>
    </visual>
  </link>
</model>

<model name="sphere_yellow">
  <pose>-30 -30 20 0 0 0</pose>
  <link name="sphere_link">
    <inertial>
      <inertia>
        <ixx>3</ixx>
        <ixy>0</ixy>
        <ixz>0</ixz>
        <iyy>3</iyy>
        <iyz>0</iyz>
        <izz>3</izz>
      </inertia>
      <mass>3.0</mass>
    </inertial ...