It sounds like the content indexing isn't running automatically as it should. Please see: Searching and Indexing Troubleshooting for possible causes and diagnostic tips.
As it says at the top of that doc, one of the most common troubleshooting procedures is rebuilding the index from scratch. That requires a restart of Confluence, so you may want to rule out some of the causes on the list before scheduling a restart.
Please let us know how it goes, and how we can help.
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