When I am spawning 2 models in Gazebo using a launch file everything works as intended, but when I try to spawn the same 2 models in the same way, they behave differently. Do you have any idea what could cause that?

From the launch file:

<group ns="$(arg robot_name)">

  <!-- load robot_description -->
  <include file="$(find ballbot_description)/launch/bb_description.launch">
     <arg name="robot_name" value="$(arg robot_name)" />
     <arg name="ground_truth" value="$(arg ground_truth)" />
     <arg name="motors_controller_type" value="$(arg motors_controller_type)" />
     <arg name="wheel_type_single" value="$(arg wheel_type_single)" />
     <arg name="imu_update_rate" value="$(arg imu_update_rate)"/>
     <arg name="control_period" value="$(arg control_period)"/>
     <arg name="use_realsense" value="$(arg use_realsense)"/>
     <arg name="use_laser" value="$(arg use_laser)"/>
  </include>

  <!-- robot spawner -->
  <node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-urdf -model $(arg robot_name) -param robot_description -x $(arg robot_x_pos)   -y $(arg robot_y_pos) -z $(arg robot_z_pos)" />

for the first model and

<!-- load ball description: -->
<group ns="$(arg ball_name)" if="$(arg use_the_ball)">
  <include file="$(find ballbot_description)/launch/ball_description.launch">
     <arg name="ball_name" value="$(arg ball_name)" />
     <arg name="wheel_type_single" value="$(arg wheel_type_single)"/>
  </include>
<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-urdf -model $(arg ball_name) -param robot_description -x $(arg ball_x_pos) -y $(arg ball_y_pos) -z $(arg ball_z_pos)" />
</group>

for the second model. This works for me (I didn't create this file, it's off GitHub)

Now in the python script I am using frobs_rl which has load and spawn functions which uses

def gazebo_spawn_urdf_path( model_path, model_name="robot1", robot_namespace="/", reference_frame="world",
                        pos_x=0.0, pos_y=0.0, pos_z=0.0, 
                        ori_x=0.0, ori_y=0.0, ori_z=0.0, ori_w=1.0) -> bool:

rospy.wait_for_service("/gazebo/spawn_urdf_model")
client_srv = rospy.ServiceProxy("/gazebo/spawn_urdf_model", SpawnModel)

if os.path.exists(model_path) is False:
    print("Error: model path does not exist")
    return False

model = xacro.process_file(model_path)
encoding = {}
model_string = model.toprettyxml(indent='  ', **encoding)

srv_model = SpawnModelRequest(  model_name=model_name,
                                model_xml=model_string,
                                robot_namespace=robot_namespace,
                                initial_pose=Pose(  position=Point(x=pos_x, y=pos_y, z=pos_z),
                                                    orientation=Quaternion(x=ori_x, y=ori_y, z=ori_z, w=ori_w)),
                                reference_frame=reference_frame)

try:
    result = client_srv(srv_model)
    return result.success, result.status_message
except ServiceException:
    print("Error: Package used in model was not found, source the workspace in all terminals.")
    return False

which I set up using

spawn_robot=True
    model_name_in_gazebo="ballbot"
    namespace="/ballbot/joints"
    pkg_name="ballbot_description"
    urdf_file="bb.xacro" 
    urdf_folder="/urdf"
    controller_file="wheel_effort_controllers.yaml"
    controller_list=["wheel1_effort_controller", "wheel2_effort_controller", "wheel3_effort_controller"]
    urdf_xacro_args=[   "robot_name:=ballbot",
                        "ground_truth:=True",
                        "motors_controller_type:=EffortJointInterface",
                        "wheel_type_single:=False",
                        "imu_update_rate:=200",
                        "control_period:=0.01",
                        "use_laser:=True",
                        "use_realsense:=True",
                        "ball_name:=ball"]
    rob_state_publisher_max_freq=100
    model_pos_x=0.0; model_pos_y=0.0; model_pos_z=0.0 
    model_ori_x=0.0; model_ori_y=0.0; model_ori_z=0.0; model_ori_w=0.0

    ros_urdf.urdf_load_from_pkg(pkg_name=pkg_name, model_name=urdf_file, folder=urdf_folder, param_name="robot_description", ns="/ballbot", args_xacro=urdf_xacro_args)

    reset_mode=2
    step_mode=1
    num_gazebo_steps = 2

    reset_controllers=True

    super(BallbotEnv, self).__init__( spawn_robot=spawn_robot, model_name_in_gazebo=model_name_in_gazebo, namespace=namespace, pkg_name=pkg_name, 
                urdf_file=urdf_file, urdf_folder=urdf_folder, controller_file=controller_file, controller_list=controller_list, 
                urdf_xacro_args=urdf_xacro_args, rob_state_publisher_max_freq= rob_state_publisher_max_freq, rob_st_term=False,
                model_pos_x=model_pos_x, model_pos_y=model_pos_y, model_pos_z=model_pos_z, 
                model_ori_x=model_ori_x, model_ori_y=model_ori_y, model_ori_z=model_ori_z, model_ori_w=model_ori_w,
                reset_mode=reset_mode, step_mode=step_mode, reset_controllers=reset_controllers, num_gazebo_steps=num_gazebo_steps)

and for the second model:

    model_name_in_gazebo="ball"
    namespace="/ball"
    pkg_name="ballbot_description"
    urdf_file="ball.xacro" 
    urdf_folder="/urdf"
    controller_file=None
    controller_list=[]
    urdf_xacro_args=["ball_name:=ball",
                     "wheel_type_single:=False"]
    rob_state_publisher_max_freq=100
    model_pos_x=0.0; model_pos_y=0.0; model_pos_z=0.0
    model_ori_x=0.0; model_ori_y=0.0; model_ori_z=0.0; model_ori_w=0.0

    ros_spawn.spawn_model_in_gazebo(pkg_name=pkg_name, model_urdf_file=urdf_file, controllers_file=controller_file, controllers_list=controller_list,
                                    model_urdf_folder=urdf_folder, ns=namespace, args_xacro=urdf_xacro_args, max_pub_freq=rob_state_publisher_max_freq,
                                    rob_st_term=False, gazebo_name="ball", gaz_ref_frame="world", pos_x=model_pos_x, pos_y=model_pos_y, pos_z=model_pos_z,
                                    ori_w=model_ori_w, ori_x=model_ori_x, ori_y=model_ori_y, ori_z=model_ori_z)

When I spawn the 2 models using python, the wheels of my robot pass straight through the ball, whereas they stay on top of it when I use the launch file.