jQuery Scroll not triggering

Solution:

The default behaviour should work. There should be some style issues by which the window’s scroll is being blocked.

 

jQuery(window).scroll(function() {
    if (jQuery(window).scrollTop() > 300) {
        jQuery(".cust_sticky_header").addClass('sticky');
    } else {
        jQuery(".cust_sticky_header").removeClass('sticky');
    }
}); 
.sticky {
  position: sticky;
  top: 0;
}
.cust_sticky_header {
  background: #ff0;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<div class="cust_sticky_header">Sticky header</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>
<div>Some content to scroll</div>