为什么我的WordPress网站的某个部分中的某些自定义字段不起作用？

I'm building a WordPress website. Everything went fine before using custom post types. When I use CPT and write PHP code then a section can't print the custom fields. This affects only one section. Other sections and custom fields works fine.

Before adding this code the custom field worked fine:

<?php
$args = array("post_type" => "project_feature");
$loop = new WP_Query( $args );
while ( $loop->have_posts() ) : $loop->the_post(); ?>

<div class="col-sm-4">

<?php
    if(has_post_thumbnail())
    {
        the_post_thumbnail();
    }
?>
    <h3><?php the_title(); ?></h3>
    <div class="black_text"><?php the_content(); ?></div>
</div>  <!-- col  -->  

<?php endwhile; wp_reset_query(); ?>

But now if I delete this code my custom feature still doesn't work. I don't understand what I am doing wrong. I still don't know if it's my code or something else in WordPress.

This custom fields within the instructor section aren't working:

<?php

// instructor custom fields

$instructor_name = get_field('instructor_name');
$instructor_description = get_field('instructor_description');
$students = get_field('students');
$views = get_field('views');
$courses = get_field('courses');

?>


<!-- INSTRUCTOR
=========================================
-->    
<section id="instructor">
    <div class="container">
        <div class="row">
            <div class="col-sm-6">
                <div class="row">
                    <div class="col-sm-8">
                        <h3>Your Instructor <small><?php echo $instructor_name; ?></small></h3>
                    </div>  <!-- col  -->
                    <div class="col-sm-4">
                        <ul>
                            <li>
                                <a href="http://www.fb.com"><i class="fa fa-facebook social" aria-hidden="true"></i></a>
                            </li>
                            <li>
                                <a href="http://www.google.com"><i class="fa fa-google-plus social" aria-hidden="true"></i></a>
                            </li>
                            <li>
                                <a href="http://www.linkedin.com"><i class="fa fa-linkedin social" aria-hidden="true"></i></a>
                            </li>
                        </ul>
                    </div>  <!-- col  --> 
                </div>  <!-- row -->     
                <div class="row">
                    <div class="col-sm-12">
                        <p class="black_text"><?php echo $instructor_description; ?></p>
                        <hr>
                    </div>  <!-- col  -->
                </div>  <!-- row -->  
                <div class="row">
                    <div class="col-sm-12">
                        <h3>The numbers <small>They don't lie...</small></h3>
                    </div>  <!-- col  -->
                    <div class="col-sm-4 round_display">
                        <p class="black_text"><?php echo $students; ?> students</p>
                    </div>  <!-- col  -->             
                    <div class="col-sm-4 round_display">
                        <p class="black_text"><?php echo $views; ?> views</p>
                    </div>  <!-- col  --> 
                    <div class="col-sm-4 round_display">
                        <p class="black_text"><?php echo $courses; ?> courses</p>
                    </div>  <!-- col  --> 
                </div>  <!-- row -->                        
            </div>  <!-- col  -->
            <div class="col-sm-6">
                <img src="<?php bloginfo('stylesheet_directory'); ?>/image/brad.png" alt="man" class="img-responsive center-block income_image"/>
            </div>  <!-- col  -->                    
        </div>  <!-- row -->
    </div>  <!-- container -->            
</section>  <!-- INSTRUCTOR  -->