Can't spawn xacro model in Gazebo

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I have a robot model in xacro format and I want to insert it in a gazebo simulation. Following similar steps with the PR2 tutorial I tried creating the following launch file:

<launch>
<!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched -->
<include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(find abb_irb14000_support)/world/abb_irb14000.world"/>
    <!-- more default parameters can be changed here -->
    <arg name="paused" value="false"/>
    <arg name="use_sim_time" value="true"/>
    <arg name="gui" value="true"/>
    <arg name="headless" value="false"/>
    <arg name="debug" value="false"/>
</include>

<param name="robot_description" command="$(find xacro)/xacro.py $(find my_model_desc)/urdf/my_model.urdf.xacro"/>

<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen" args="-urdf -model my_model -param robot_description"/>  
</launch>

Although gazebo opens with no errors and the model name is displayed on the left toolbar, nothing shows inside the simulation environment and the model has no links or whatsoever. What's wrong?