Here are the possible reasons an embedded tree does not appear:

Most Common Issue

If the containing web page has a URL that begins with https, the embed code for your Zingtree must use URLs that also begin with https.

Occasionally Seen

In some cases, trees with script code in them or imported from other formatted sources may cause errors in rendering your trees.  You can track these errors by examining the Console tab using your browser's developer tools. (F12 in chrome).