テンプレート階層（上書きされない）

wordpressのテンプレート階層についての質問です。
single.phpで上書きができずにindex.phpが適用、表示されます。
原因が分かりません。

index.php⇓

php
1<!DOCTYPE html>
2<html lang="ja">
3
4<head>
5
6  <meta charset="utf-8">
7  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
8
9  <!-- Bootstrap core CSS -->
10  <link href="<?php echo get_template_directory_uri(); ?>/vendor/bootstrap/css/bootstrap.min.css" rel="stylesheet">
11
12  <!-- Custom fonts for this template -->
13  <link href="vendor/fontawesome-free/css/all.min.css" rel="stylesheet" type="text/css">
14  <link href='https://fonts.googleapis.com/css?family=Lora:400,700,400italic,700italic' rel='stylesheet' type='text/css'>
15  <link href='https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800' rel='stylesheet' type='text/css'>
16
17  <!-- Custom styles for this template -->
18  <link href="<?php echo get_template_directory_uri(); ?>/css/clean-blog.min.css" rel="stylesheet">
19
20  <?php wp_head(); ?>
21</head>
22
23<body>
24
25  <!-- Navigation -->
26  <nav class="navbar navbar-expand-lg navbar-light fixed-top" id="mainNav">
27    <div class="container">
28      <a class="navbar-brand" href="<?php echo home_url(); ?>"><?php bloginfo('name'); ?></a>
29      <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarResponsive" aria-controls="navbarResponsive" aria-expanded="false" aria-label="Toggle navigation">
30        Menu
31        <i class="fas fa-bars"></i>
32      </button>
33      <div class="collapse navbar-collapse" id="navbarResponsive">
34        <ul class="navbar-nav ml-auto">
35          <li class="nav-item">
36            <a class="nav-link" href="index.html">Home</a>
37          </li>
38          <li class="nav-item">
39            <a class="nav-link" href="about.html">About</a>
40          </li>
41          <li class="nav-item">
42            <a class="nav-link" href="post.html">Sample Post</a>
43          </li>
44          <li class="nav-item">
45            <a class="nav-link" href="contact.html">Contact</a>
46          </li>
47        </ul>
48      </div>
49    </div>
50  </nav>
51
52  <!-- Page Header -->
53  <header class="masthead" style="background-image: url('<?php echo get_template_directory_uri(); ?>/img/home-bg.jpg')">
54    <div class="overlay"></div>
55    <div class="container">
56      <div class="row">
57        <div class="col-lg-8 col-md-10 mx-auto">
58          <div class="site-heading">
59            <h1>Clean Blog</h1>
60            <span class="subheading">A Blog Theme by Start Bootstrap</span>
61          </div>
62        </div>
63      </div>
64    </div>
65  </header>
66
67  <!-- Main Content -->
68  <div class="container">
69    <div class="row">
70      <div class="col-lg-8 col-md-10 mx-auto">
71        <?php while (have_posts()) : the_post(); ?>
72          <div class="post-preview">
73            <a href="<?php the_permalink(); ?>">
74              <h2 class="post-title">
75                <?php the_title(); ?>
76              </h2>
77              <h3 class="post-subtitle">
78                <?php the_excerpt(); ?>
79              </h3>
80            </a>
81            <p class="post-meta">Posted by
82              <?php the_author(); ?>
83              on <?php the_time("Y-m-d"); ?></p>
84          </div>
85          <hr>
86        <?php endwhile; ?>
87
88        <!-- Pager -->
89        <div class="clearfix">
90          <?php echo paginate_links(); ?>
91        </div>
92      </div>
93    </div>
94  </div>
95
96  <hr>
97
98  <!-- Footer -->
99  <footer>
100    <div class="container">
101      <div class="row">
102        <div class="col-lg-8 col-md-10 mx-auto">
103          <ul class="list-inline text-center">
104            <li class="list-inline-item">
105              <a href="#">
106                <span class="fa-stack fa-lg">
107                  <i class="fas fa-circle fa-stack-2x"></i>
108                  <i class="fab fa-twitter fa-stack-1x fa-inverse"></i>
109                </span>
110              </a>
111            </li>
112            <li class="list-inline-item">
113              <a href="#">
114                <span class="fa-stack fa-lg">
115                  <i class="fas fa-circle fa-stack-2x"></i>
116                  <i class="fab fa-facebook-f fa-stack-1x fa-inverse"></i>
117                </span>
118              </a>
119            </li>
120            <li class="list-inline-item">
121              <a href="#">
122                <span class="fa-stack fa-lg">
123                  <i class="fas fa-circle fa-stack-2x"></i>
124                  <i class="fab fa-github fa-stack-1x fa-inverse"></i>
125                </span>
126              </a>
127            </li>
128          </ul>
129          <p class="copyright text-muted">Copyright &copy; Your Website 2019</p>
130        </div>
131      </div>
132    </div>
133  </footer>
134
135  <!-- Bootstrap core JavaScript -->
136  <script src="<?php echo get_template_directory_uri(); ?>/vendor/jquery/jquery.min.js"></script>
137  <script src="<?php echo get_template_directory_uri(); ?>/vendor/bootstrap/js/bootstrap.bundle.min.js"></script>
138
139  <!-- Custom scripts for this template -->
140  <script src="<?php echo get_template_directory_uri(); ?>/js/clean-blog.min.js"></script>
141
142  <?php wp_footer(); ?>
143</body>
144
145</html>