Running two subscribers concurrently in the same node

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I am working on a problem and need to use the most recent pose from amcl/future (my self-designed topic) and publish it as a tf between two frames.

I made two subscribers for: amcl_pose/future and clock. The subscriber for amcl_pose/future updates a pose struct inside my code whenever it receives a new pose estimate from amcl; the subscriber for clock ignores the clock message and publish the most recent pose from the structure to tf (basically use it to publish tf in a high rate).

The problem is that the amcl_cb function is never called by ROS. I believe it might be a concurrency issue because the two callbacks share the same data structure. What is the "correct" way to implement this? My code snippet is provided.

*Note: the pose published is not the amcl estimated pose. * *The parent_frame and child_frame are only to simplify the snippet, so it is not related. *

void amcl_cb(const geometry_msgs::PoseWithCovarianceStamped::ConstPtr& p){
    pose.header = p->header;
    pose.pose = p->pose;
}

void clock_cb(const rosgraph_msgs::Clock::ConstPtr& dummy){
    tf::TransformBroadcaster broadcaster;
    broadcaster.sendTransform(
    tf::StampedTransform(
    tf::Transform(
      tf::Quaternion(pose.orientation.x, pose.orientation.y, pose.orientation.z, pose.orientation.w), 
      tf::Vector3(pose.position.x, pose.position.y, pose.position.z)),
    ros::Time::now(),"parent_frame", "child_frame"));
}

int main(int argc, char** argv){
    ros::init(argc, argv, "robot_tf_publisher");
    ros::NodeHandle n;
    ros::Subscriber amcl_sub = n.subscribe<geometry_msgs::PoseWithCovarianceStamped>("amcl_pose/future",1000, amcl_cb);
    ros::Subscriber clock_sub = n.subscribe<rosgraph_msgs::Clock>("clock",1000, clock_cb);
    ros::spin();
}