ROS2 cmd_vel subscriber & odometry publisher - subscription callback never called

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I am writing a motor controller package in C++ that subscribes to /cmd_vel and publishes to /odom. The node runs correctly and rqt_graph shows the subscription and publishers are connected. The problem is the callback function for /cmd_vel subscriber never gets called at all. The run() function is called in the constructor, which results in odom_loop() being called to calculate and build odometry messages. The cmdvel_loop() function just has code used for testing in it. Is the while loop in the run() function causing the callback function to never be called? Does this require a multithreaded executor to handle my run() loop that publishes odometry and another thread to handle the /cmd_vel subscription callback? Or would simply moving the run() function call into the sub callback and removing the while loop fix the issue?

I have the code that works with ROS1, and I used the general idea of that for this version. The code is 500 lines long so I will show the general structure:

namespace mc{
mc::mc : Node("driver")

//  set parameters & initialize
 variables
   run()
   }
   void mc::cmdvel_callback(const
   geometry_msgs::msg::Twist::SharedPtr
   twist_msg) 
   {
   // wheel speed (m/s)
   float right_speed = twist_msg->linear.x + track_width *
  twist_msg->angular.z / 2.0;
   float left_speed = twist_msg->linear.x - track_width *
 twist_msg->angular.z / 2.0;

   // send motor commands to mc

   }
   void mc::cmdvel_loop()
   {
//test code
   }
   int mc::run(){
   // connect to serial port
   while (rclcpp::ok())
   {

       cmdvel_loop();
       odom_loop(); // reads encoder count & publishes odometry




    }
}
return 0;
{
int main(int argc, char* argv[])
{
rclcpp::init(argc, argv);
rclcpp::executors::SingleThreadedExecutor exec;
rclcpp::NodeOptions options;
auto node = std::make_shared<mc::mc>();
exec.add_node(node);
exec.spin();

rclcpp::shutdown();

return 0;
}