You are performing a strict comparison === so the letter case is important.

if(linkText === "Show more"){

How about removing the class that hides those elements?

$(".show-more a").on("click", function() {
    if(linkText === "Show more"){
        linkText = "Show less";
        $('li.hidden').removeClass('hidden')
    } else {
        linkText = "Show more";
        $('li.hidden').addClass('hidden')
    };

    $this.text(linkText);
});

jsFiddle with some additional refactors: https://jsfiddle.net/rt0cs8jh/

You can assign a hidden class to every element you want to hide by default. Then remove this class or just slide it on button click.

var active = false;
$(".show-more a").on("click", function() {
  $('li.hidden').slideToggle();
  $(this).text(active ? 'Show more' : 'Show less')
  active = !active;
});

.hidden {
  display: none;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<ul>
  <li>Lorem</li>
  <li>Lorem</li>
  <li class='hidden'>Lorem</li>
  <li class='hidden'>Lorem</li>
  <li class='hidden'>Lorem</li>
  <li class="hidden">Ipsum</li>
</ul>
<div class="show-more">
  <a href="#">Show more</a>
</div>

here's how I would write it

$(".show-more a").on("click", function() {
    var currentText = $(this).text();
    if(currentText === "Show More"){
        $('.hidden').show();
        $(this).text("Show Less");
    } else {
        $('.hidden').hide();
        $(this).text("Show More");
    };
});