ROS pointcloud assembler service call not returning pcd

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Hi I am trying to publish pointclouds that have been most recently entered into the pointcloud assembler http://wiki.ros.org/laser_assembler#point_cloud_assembler However i only get PCD when last_time = rospy.Time(0,0). The point cloud assembler service call is meant to give pcd which is between a begin and an end time. Entering rospy.Time(0,0) into the begin field will give back everything in the buffer. But I only want PCD that is newly entered... But the following code does not return any PCD.

last_time = rospy.Time(0,0)
rate = rospy.Rate(0.5)
while not rospy.is_shutdown():
    try:
        rospy.wait_for_service("assemble_scans")
        assemble_scans = rospy.ServiceProxy('assemble_scans', AssembleScans)
        resp = assemble_scans( last_time, rospy.get_rostime())
        last_time = rospy.get_rostime()
        pub.publish(resp.cloud)                     
    except rospy.ServiceException, e:
        print "Service call failed: %s"%e
    rate.sleep()