I originally made this post, thinking that my problem was specific to the Ninja Forms plugin (the original post was obviously put on hold, for being off-topic). My reason for thinking so, was that WordPress' native gallery shortcode was working correctly, but it would seem that the issue affects all plugin shortcodes.
As this is no longer an issue with a specific plugin, but WordPress's ability to execute plugin shortcodes -- --, I've taken the liberty to assume that it is now considered on-topic.
Even with an empty functions.php file and the following as my only code, the issue still persists.
<?php if (have_posts()) : while (have_posts()) : the_post(); the_content(); endwhile; endif; ?>
When inspecting the page, this appears to output some of what it's supposed to, in the form of a script or div, but it's as if the plugins are never rendered, despite being initialized.
I've been stuck here for a few days now, so if anyone could give me a push in the right direction, that would be amazing!
Note: I apologize if this is still considered to be off-topic. In case it is, please disregard this post.