Subscriber Callback not being triggered when JointState message is passed

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Dear all,

I have come across a strange problem; maybe it's just something simple, but I am not really good at ROS programming.

Explaining the problem: I am using the rosserial package on Arduino Mega to log data and apply control inputs to a set of servos.

In order to do that, I created a subscriber as follows:

void controlCallback(const sensor_msgs::JointState& servo_des_states);
ros::Subscriber<sensor_msgs::JointState> control_sub("controlInput", controlCallback);

void setup(){
  ...
  nh.subscribe(control_sub);
  ...
}

...
void controlCallback(const sensor_msgs::JointState& servo_des_states){

    servoRadians = servo_des_states.position;
    sendControlInput(RadToPWM(servoRadians), controlSpeed);
    digitalWrite(13, HIGH-digitalRead(13));
}

when invoking the serial_node.py it lists that the subscriber is active and listening on the right topic:

cr055@cr055:~/catkin_ws$ rosrun rosserial_python serial_node.py /dev/ttyACM0 _baud:=115200
[INFO] [1548676899.466166]: ROS Serial Python Node
[INFO] [1548676899.471357]: Connecting to /dev/ttyACM0 at 115200 baud
[INFO] [1548676901.575863]: Requesting topics...
[INFO] [1548676904.505507]: Note: publish buffer size is 512 bytes
[INFO] [1548676904.505995]: Setup publisher on /tf [tf/tfMessage]
[INFO] [1548676904.509778]: Note: subscribe buffer size is 1024 bytes
[INFO] [1548676904.510289]: Setup subscriber on controlInput [sensor_msgs/JointState]

Although it seems everything correctly set up, the callback does not get triggered when I publish the mentioned messages.

If I try to send another kind of message (redefining the subscriber type and callback), for instance Float32, it works flawlessly.

It seems that the problem comes out when I use more complex kinds of messages (like sensor_msgs or MultiArray).

I hope someone has any clue about what's wrong here.

TL;DR: the callback function in the subscriber does not get invoked if the message type is multidimensional like JointState or Float32MultiArray. If I use simple floats I get the callback working.

UPDATE which might help: if I publish only a vector of two elements e.g. {position:[0,0]} the callback works. Am I missing something in the declaration of the published message?

UPDATE #2: I think that the arduino serial cannot handle packages as big as JointState if I publish 18 servos all together. Still, a workaround / alternative to publish 18 float values is welcome.

Sincerely,

Alessandro