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The std_msgs/Float32 message type has no header and hence no time stamp. As a result, time synchroniser will not work. See the documentation: http://docs.ros.org/api/std_msgs/html/msg/Float32.html

The reason why the code from the previous answer works is because sensor_msgs/PointCloud2 does have a header (and time stamp - http://docs.ros.org/api/sensor_msgs/html/msg/PointCloud2.html).

It's a bit of a late answer, but I hope it helps.