Cartographer submap problem

When I use Cartographer to create a map, my submaps only save maybe 3 or 4. It cause that when the range of map need more submap to create global map, the global map will miss a part because my previous submaps disappear. I call the topic to check the submap index. Find that the previous submap index cannot check in the topic. If sombody couid give me some suggestion, I will appreciate you.

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after