Client doesn't return when declared inside c++ class in ROS 2

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Hi, I need to declare a client inside my class so I can access various services within this class' execution, but everytime I use the client I'm stuck waiting for a response(specifically waiting for "resp.wait()" function) . I've use this exact same methods but outside a class declaration in another node to call services and it has worked. My code is as follows:

#include "rclcpp/rclcpp.hpp"
#include "cstm_msg/srv/mov_loc.hpp"
class ModuloCentral: public rclcpp::Node
{
public:
  ModuloCentral() : Node("modulo_central")
  {
      mov_loc_client = this->create_client<cstm_msg::srv::MovLoc>("mov_localiz");
      while (!mov_loc_client->wait_for_service(1s)) {
          if (!rclcpp::ok()) {
              RCLCPP_ERROR(rclcpp::get_logger("modulo_central"), "Interrupted while waiting .");
              exit(0);
       }
          RCLCPP_INFO(rclcpp::get_logger("modulo_central"), "service not available, waiting again...");
      }
  }

  cstm_msg::srv::MovLoc::Response::SharedPtr move_robot(RobPos pos)
  {
      auto request = std::make_shared<cstm_msg::srv::MovLoc::Request>();
      for (size_t i = 0; i < 8; i++)
      {
           request->pose[i] = pos[i];
      }
      auto resp = mov_loc_client->async_send_request(request);
      printf("esperando respuesta de mov\n");
      resp.wait();
      printf("results are in! \n");
      return resp.get();
  }
private:
  void start()
  {
      auto r = move_robot(posQR1);
  }
   rclcpp::Client<cstm_msg::srv::MovLoc>::SharedPtr mov_loc_client;

};
int main(int argc, char * argv[])
{
   rclcpp::init(argc, argv);
   rclcpp::spin(std::make_shared<ModuloCentral>());
   rclcpp::shutdown();
   return 0;
}

I know my server works as I've tested them it with other clients (not written inside a class) and with "ros2 service call" in the terminal getting a proper response each time. I know the request is processed by the server when I call it like this but after that the response still doesn't reach this node. Finally to call the function start I've tried putting a timer in the constructor with the function as a callback, call it in the at the end of the constructor method, and calling it with a subscription. It doesn't seem to change the outcome. I haven't been able to find any examples of a client defined within a class has anyone else here tried it? please help :(

ps. my code isn't exactly like this but this are the parts regarding clients

update 1: I've tried using other clients on other classes (nodes) and the reaction is exactly the same (service is being executed but no answer comes back)

update 2: I've tried calling start like this:

from within the constructor:

  ModuloCentral() : Node("modulo_central")
  {
    /.../ 
    start()
  }

As a callback to a ROS timer:

  ModuloCentral() : Node("modulo_central")
  {
      /.../ 
      timer_ = create_wall_timer(
      500ms, std::bind(&ModuloCentral::start, this));
  }
 /.../
 private:
    rclcpp::TimerBase::SharedPtr timer_;

And as a callback to a subscriber:

  ModuloCentral() : Node("modulo_central")
  {
      subscription_ = this->create_subscription<std_msgs::msg::Bool>(
      "start_central", 10, std::bind(&ModuloCentral::start_callback, this, _1));
  }
  void start_callback(const std_msgs::msg::String::SharedPtr msg)
  {
     start();
  }

update 3: Acording to the answer my client and start callback where in the same mutually exclusive callback group. So to solve this I declared a new mutually exclusive callback group where I added all my clients (I only want to call one service at a time) then I added the node to a multi threaded executor and spin the executor. The final code ended up ... (more)