View previous using posts_nav_link() works only for 9 previous posts

My blog is located at https://www.toastyfrog.net. I am using the posts_nav_link tag in my index.php page to allow the user to view previous posts, as follows:

<?php posts_nav_link('','','« Older Entries') ?>

It works fine, as long as you only step through the previous 9 postings (starting from https://www.toastyfrog.net/page/1/ and clicking through to https://www.toastyfrog.net/page/10/). If you click one more time in an attempt to get to the 10th one (https://www.toastyfrog.net/page/11/), you get a 404 error.

I don’t have nearly enough WP expertise to know how to troubleshoot this consistently inconsistent sort of problem, so I’m hoping someone can help me with it!