ApproximateTimeSynchronizer does not trigger the callback function

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I have checked with some related problems. Most of them are using the message_filter directly instead of approximateTimeSychronizer. However, even though I use the ApproximateTimeSynchronizer, my callback function can still not be called.

Specifically, I initially let my ros node subscribe two topic: /odom and /pc (PointCloud2). It works prettry well. I use another node to convert /pc into /aligned_pc, and let the previous node subscribe to /aligned_pc instead. It cannot work.

void callback(const OdometryConstPtr& odom, const PointCloud2ConstPtr& pc){
      whatever...
}

Below is my main function

 int main(int argc, char** argv){
    ros::init(argc, argv, "sync");
    //planner p;

    ros::NodeHandle nh;
    message_filters::Subscriber<nav_msgs::Odometry> odom_sub(nh, "odom", 100);
    message_filters::Subscriber<sensor_msgs::PointCloud2> pc_sub(nh, "aligned_pc", 100);

    typedef sync_policies::ApproximateTime<Odometry, PointCloud2> MySyncPolicy;
    Synchronizer<MySyncPolicy> sync (MySyncPolicy(10), odom_sub, pc_sub);
    sync.registerCallback(boost::bind(&callback, _1, _2));

    ros::spin();
}