New elements are indexed whenever it is published. Make sure that this option is enabled (it should be enabled by default): https://i.imgur.com/E4XQiMt.png
If you use some sort of import tool to add the posts, that might not trigger the index refresh. You can also enable the wp_cron() option to index new contents every hour as well: https://i.imgur.com/wCK4BCB.png
It will look for missing elements in every hour, and extend the index if needed.
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