What are possible reasons for empty service responses?

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Hey, I have a problem regarding a service call. My code is very similar to the example given below. This example WORKS WELL, I am aware of that. However, in my very long code I have the problem that in a similar situation, the response in printer.cpp is empty after the service call. The service call itself is executed and the result is calculated, but the response object after the call is empty. That is very strange as I give the request and response as references and thus it should work on the given object. I would like to provide you with the relevant code, but I have not a single clue where the issue lies and thus would have to paste everything here, which would be way too much.

So, basically, I would like to know what possible reasons there are for a service response to be empty/uninitialized. Any help is appreciated.

calcu.cpp

#include "ros/ros.h"
#include "std_msgs/String.h"
#include "beginner_tutorials/AddTwoInts.h"
#include <sstream>

bool calcuCallback(beginner_tutorials::AddTwoInts::Request &req,
                    beginner_tutorials::AddTwoInts::Response &res)
{
    res.sum = req.a + req.b;
}
int main (int argc, char** argv)
{
    ROS_INFO("calcu running");
    ros::init (argc, argv, "calcu");
    ros::NodeHandle nh;
    // create a ROS service
    ros::ServiceServer service= nh.advertiseService("add_two_ints", &calcuCallback);
    // spin
    ros::spin();
    return (0);
}

printer.cpp

#include <ros/ros.h>
#include <opencv2/opencv.hpp>
#include "beginner_tutorials/AddTwoInts.h"
#include "beginner_tutorials/Num.h"

void numCallback(const beginner_tutorials::NumPtr msg)
{
    static beginner_tutorials::AddTwoInts::Request request;
    static beginner_tutorials::AddTwoInts::Response response;
    request.a = msg->num;
    request.b = 3;
    ros::service::call("add_two_ints", request, response);
    std::cout << "Response sum = " << response.sum << "\n"; // RESPONSE WOULD BE EMPTY/UNINITIALIZED
}
int main (int argc, char** argv)
{
    ROS_INFO("printer running");
    ros::init (argc, argv, "printer");
    ros::NodeHandle nh;
    if(!ros::service::waitForService("add_two_ints",10000)) return -1;
    // create ROS subscribers for local landmarks and dGPS pose estimates
    ros::Subscriber sub = nh.subscribe("inputs", 1, &numCallback);
    // spin
    ros::spin();
    return (0);
}

addTwoInts.srv

int64 a
int64 b
---
int64 sum

Num.msg

int64 num