When you add new books to your Kindle, it indexes them to make searches faster.
Sometimes a particular ebook gets corrupted, and the indexing process gets stuck. If you do a search for some gibberish , i.e., “ngfjksg” while it indexes it will show the status of the indexing. This is helpful to see if the indexing is stuck. If it is, delete the ebook, and resend it to the Kindle.