It's a similar issue to 16390193, but I still can't figure out how to get it to work (I'm not a web developer...). My test site is http://testooop.tumblr.com/ (based on copycat theme). Here are some theme codes (that I think are relevant to the problem).
<body class="{block:IndexPage}index-page{/block:IndexPage}{block:PermalinkPage}permalink-page{/block:PermalinkPage} layout-{select:Max Columns}">
{block:IndexPage}
<div id="backToTop" class="back-to-top" title="Back to top">
<i class="fa fa-2x fa-angle-up"></i>
</div>
{/block:IndexPage}
Post mark-ups
{block:Posts}
<article class="post {PostType}{block:PermalinkPage} {block:Date}not-{/block:Date}page{/block:PermalinkPage}">
JS
<script type="text/javascript">
var copycat = {{block:IfInfiniteScrolling}infScroll: true,{/block:IfInfiniteScrolling}{block:IfNotInfiniteScrolling}infScroll: false,{/block:IfNotInfiniteScrolling} {block:IndexPage}indexPage: true{/block:IndexPage}{block:PermalinkPage}indexPage: false{/block:PermalinkPage}, prefill: false};
</script>
<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
<script type="text/javascript" src="//static.tumblr.com/anzluor/z2tn74e8h/masonry.pkgd.min.js"></script>
{block:IfInfiniteScrolling}<script type="text/javascript" src="//static.tumblr.com/anzluor/ycPn74dop/jquery.infinitescroll.min.js"></script>{/block:IfInfiniteScrolling}
<script type="text/javascript" src="//static.tumblr.com/anzluor/Jwrn7rcyd/copycat.min.js"></script>
Not sure whether this info is sufficient, though. Thank you in advance!