You almost certainly have one book which is failing to index - this will prevent any new books from showing up.
Delete the new books you've added, and then add them a few at a time, waiting for each batch to finished indexing before you add the next.
To see what is currently indexing, go to the home page, tap the "Search" button, and enter a random text string. It will say "N items not currently indexed" (if any); tap on that link to see what those items are.
One thing I would say: you really should be converting books to AWZ3 format, not Mobi; there's no reason to use Mobi.
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