My older posts disappear when new posts are added – please help!!!

https://codex.www.ads-software.com/Next_and_Previous_Links

Thanks for the website. The website mentions the following:

“[the next/back links] is often found in a paragraph code or a division like this:

<div class=”navigation”><p><?php posts_nav_link(); ?></p></div>

The parameters of the tag are as follows:

<?php posts_nav_link(‘separator’,’prelabel’,’nextlabel’); ?>”

Where can these codes be found? Is there a possibility that these codes are missing from my site?

some themes use the code – some don’t

In which section of my theme would I need to add the code to?

likely your theme’s single.php