publishers two message on the same topic not working for subscriber

edit

Hi everyone; I am sending two deferent data message publishers on the same topic, and I see both data publisher in output when I use echo. I use two node and I make the subscriber from this publisher I see just the last message publish. Always loses the first message and stay the last. M y code is:

  output_modem_publisher_ = nh_.advertise<std_msgs::String>(modem_topic_, 100);
  modemin_subscriber_ = nh_.subscribe(modem_in_, 100, &Depth::modeminCallback, this);

void Depth::callPublisher()
{
    std::string out_modem2;
    std::stringstream monsun2;
    monsun2 << 2 << "@" << "d";
    out_modem2 = monsun2.str();
    std_msgs::String m2;
    m2.data = out_modem2;
    output_modem_publisher_.publish(m2);    


    std::string out_modem3;
    std::stringstream monsun3;
    monsun3 << 3 << "@" << "d" ;
    out_modem3 = monsun3.str();
    std_msgs::String m3;
    m3.data = out_modem3;
    output_modem_publisher_.publish(m3);    
}
 ...
 ...

  ros::Rate loop_rate(15);
  int count = 0;
  while (ros::ok())
  {
    depth.callPublisher();
    ros::spinOnce();
    loop_rate.sleep();
    ++count;
  }

The echo results are for the publisher:

rostopic echo depth/modem_topic 

data: 2@d
---
data: 3@d
---
data: 2@d
---
data: 3@d
---

... The echo results are for the subscriber :

rostopic echo depth2/modem_in:


    data: 3@d
    ---
    data: 3@d
    ---
    data: 3@d
    ---
    ...

when I use usleep(ms) for 10ms I see both data by subscriber. But I don't like to use usleep() due to make low performance for my program.

the code of subscriber is:

void DepthMonsun::modeminCallback(const std_msgs::StringPtr& modemin_msg)
{
//  ROS_INFO(" modeminCallback ");

  std::string in_modem;
  in_modem = modemin_msg->data;
  std::vector<std::string> v;
  split(in_modem, v, '@');
    //decoder the data that received from modem/out of MONSUN

    if (in_modem[0] == '2')
    {
      std::vector<std::string> v;
      split(in_modem, v, '@');
      x = 0;
      x = atof(v.at(2).c_str());
....
    }
else if (in_modem[0] == '3')
{
      std::vector<std::string> v;
      split(in_modem, v, '@');
      y = 0;
      y = atof(v.at(2).c_str());
...
}

Any idea for solving this problem...,

thanks