Robot model is completely grey when spawned in gazebo

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I am working on Gazebo 9 with ROS Melodic in Ubuntu 18.04. I have a very simple urdf with 6 links: the robot has no actuated joints and is entirely passive. It is to be said that the model is not a single kinematic chain.

To provide joint feedback I added the following in my robot model:

<gazebo>
    <plugin name="joint_state_publisher" filename="libgazebo_ros_joint_state_publisher.so">
            <robotNamespace>/${name}</robotNamespace>
            <jointName>${name}_joint_1, ${name}_joint_2, ${name}_joint_3</jointName>
            <updateRate>10.0</updateRate>
            <alwaysOn>true</alwaysOn>
    </plugin>
</gazebo>

The problem is that when I spawn it in gazebo using urdf_spawner it seems to be simulated because the links move according to gravity and the joint states are published in ros, but the entire robot is opaque grey!!!

Some useful details:

• The meshes I use might be slightly heavy (I am also scaling them), but I tried to spawn then individually using a simple test world file and they look fine.
• I am using the meshes themselves for collisions... Is this bad?
• In the urdf I am using for the joints <dynamics friction="0.01" damping="0.01"/> which might not be precise.
• I am not entirely sure about the values I gave for effort="..." velocity="..." in the limit tag of the joints in the urdf.
• I am not defining any transmission as the robot is entirely passive.

What could be the problem? Please tell me if you need additional information.

Thank you so much in advance.