How to handle the dropped messages in approximate time syncronizer.

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I have a topic published with images from a camera and another topic with details of point cloud. When I plot these two topics, there is a time lag between the two plots. I am using approximate time sync to handle this time lag. But there is a drop in the frame rate as the approximate time sync algorithm is dropping some messages. Is there any method to handle this dropped messages?

Please help me to get a solution.