Show all posts in categories

With the help of the Template Hierarchy article, determine what Template is displaying your Category archives as you may need to edit that file based on your findings.

You probably have your Administration > Settings > Reading, Blog pages show at most set to 2, so your Category template may display that number of posts, and doesn’t have the next_posts_link and previous_posts_link in use to show navigation to the rest of the posts in that category. You may need to look at how the WordPress Default theme handles that and fix your category template accordingly.