C++ plugin cannot get parameter that has been uploaded to param server later

edit

I want to load parameters from the ROS param server in the Gazebo model plugin. I do not have any problem with getting the parameters thet were set in the launch files, but I run a node that uploads a list of parameters to the rosparam server later in the run. I can see and read the parameters using terminal, but the gazebo plugin does not see the new parameters (I check for them inside the plugin on every update).

Can you provide any insight into why is this happening?

EDIT

I am accessing the parameters from gazebo plugin using

std::string param_name;
this->rosNode->getParam("/param_name", param_name);

EDIT 2

code snippet

namespace gazebo {
  // Constructor
  MyPlugin::MyPlugin() {}

  // Default plugin init call.
  void MyPlugin::Init() {}

  void MyPlugin::Load(physics::ModelPtr _model,
                                  sdf::ElementPtr _sdf) {

    this->updateConnection = event::Events::ConnectWorldUpdateBegin(
            boost::bind(&MyPlugin::OnUpdate, this));

    // Initialize ros, if it has not already been initialized.
    std::string node_name = std::string("my_node");
    if (!ros::isInitialized()) {
      int argc = 0;
      char **argv = nullptr;

      ros::init(argc, argv, node_name,
                ros::init_options::NoSigintHandler);
    }

    // Create ROS node.
    this->rosNode.reset(new ros::NodeHandle(node_name));

    ros::AsyncSpinner spinner(2); // Use 2 threads
    spinner.start(); // spin() will not return until the node has been shutdown
  }


  void MyPlugin::loadParameters()
  {
    this->parameters_loaded = true;
    std::string parameter_from_launch, parameter_from_node;
    if (! this->rosNode->getParam("/parameter_from_launch", parameter_from_launch))
    {
      gzmsg << "ROS parameter '/parameter_from_launch' was not found \n";
      this->parameters_loaded = false;
    }
    gzmsg << "Parameter from launch: " << parameter_from_launch << "\n";
    if (! this->rosNode->getParam("/parameter_from_node", parameter_from_node))
    {
      gzmsg << "ROS parameter /parameter_from_node was not found. \n";
      this->parameters_loaded = false;
    }
    gzmsg << "Parameter from node: " << parameter_from_node << "\n";

  }


  void MyPlugin::OnUpdate() {
    if(!this->parameters_loaded){
      MyPlugin::loadParameters();
    }
  }
}

the parameter_from_launch is found on the first try and prints the correct value. The "ROS parameter /parameter_from_node was not found. \n" keeps being printed and the gzmsg << "Parameter from node: " << parameter_from_node << "\n" returns an empty string