message_filters::sync::ApproximateTime does not call the callback

edit

I am trying to subscribe to a pointcloud and it's normal from to different subscribers and pass it on to a common function via approximate time sync. Please find minimal code below:

void OrganizedMultiPlaneSegmentation::onInit() {
  sub = private_nh.subscribe("/cloud/cloud_out_merged", 1,
                             &OrganizedMultiPlaneSegmentation::segment, this,
                             ros::TransportHints().tcpNoDelay());
}

void OrganizedMultiPlaneSegmentation::segment(
    const sensor_msgs::PointCloud2& msg) {
  ros::NodeHandle& nh_ = getNodeHandle();

  sub1_.subscribe(nh_, "/cloud/cloud_out_merged", 5);
  if (sub1_.getSubscriber().getNumPublishers() == 1) {
    // NODELET_INFO("Got a subscriber to scan, starting subscriber to merged
    // pointcloud");
  }
  sub2_.subscribe(nh_, "/cloud_out_normal", 5);
  if (sub2_.getSubscriber().getNumPublishers() == 1) {
    // NODELET_INFO("Got a subscriber to scan, starting subscriber to normal
    // pointcloud");
  }
  typedef sync_policies::ApproximateTime<sensor_msgs::PointCloud2,
                                         sensor_msgs::PointCloud2>
      MySyncPolicy;
  Synchronizer<MySyncPolicy> sync(MySyncPolicy(40), sub1_, sub2_);
  //   sync.setAgePenalty(1.0);
  sync.registerCallback(boost::bind(
      &OrganizedMultiPlaneSegmentation::segmentPlanes, this, _1, _2));
}

// Callback function to be called
void OrganizedMultiPlaneSegmentation::segmentPlanes(
    const PointCloud2ConstPtr& cloud_msg1,
    const PointCloud2ConstPtr& cloud_msg2) {
  pcl::PointCloud<pointT>::Ptr cloud(new pcl::PointCloud<pointT>());
  pcl::fromROSMsg(*cloud_msg1, *cloud);
  pcl::PointCloud<pcl::Normal>::Ptr normalCloud(
      new pcl::PointCloud<pcl::Normal>());

I have tried changing queue size of the subscriber, the buffer size of the policy, and both don't seem to work. Please let me know if there is something obvious that I am doing wrong,or what can be possible causes and remedies for this. Happy to provide more information and thank you!