Python ApproximateTimeSynchronizer not working

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Dear all,

I want to subscribe to synchronized depth and rgb images using message_filters and ApproximateTImeSynchoronizer.

def rgb_callback(image):
     rospy.loginfo("got RGB image")

def depth_callback(image):
    rospy.loginfo("got DEPTH image")

def scan_callback(image,depth):
    rospy.loginfo("got synched images")


def hand_tracker():

    rospy.init_node('hand_tracker')
    image_sub=message_filters.Subscriber('/kinect2/hd/image_color_rect',Image)
    depth_sub=message_filters.Subscriber('/kinect2/hd/image_dept_rect',Image)        

    rospy.Subscriber('/kinect2/hd/image_color_rect',Image,rgb_callback)    
    rospy.Subscriber('/kinect2/hd/image_depth_rect',Image,depth_callback)    

    ts=message_filters.ApproximateTimeSynchronizer([image_sub,depth_sub],10,1)    
    ts.registerCallback(scan_callback)
    try:
        rospy.spin()
    except KeyboardInterrupt:        
        print "Shutting down"

The problem is that rgb callback and depth callback are called, but the scan_callback using ApproximateTimeSynchronizer not. Is it a bug or is there something I am doing wrong?