Problems with NEXT and PREVIOUS tags

So I’ve got some problems with the next_posts_link and previous_posts_link actually going to the right entry on a web site – https://www.helengoesto.com/wp/

The photos are entries in their own category, and I have used this line of code in the index.php file to block out other categories from displaying there:

<?php c2c_get_random_posts($num_posts = 1,
$format = "%post_content%",
$categories = '2 3 4 6',
$order = 'DESC',
$date_format = '',
$authors = '',
$include_passworded_posts = false); ?>

Then, where the written entry is (which is a different category), I have the usual tags for displaying content within the Loop. I am using the default tags for PREVIOUS and NEXT:

<?php next_posts_link('&laquo; Previous Entries') ?>
<?php previous_posts_link('Next Entries &raquo;') ?>

… but it doesn’t work. Every time I click the “Previous Entry” link, the next page displayed is the same written content.

Again, the temporary site is https://www.helengoesto.com/wp/ — if you want to try testing out what I’ve attempted to describe here. I’m going to continue trying to work it out, but am in need of someone’s help right now …

Thnx!