I experienced the same problem, the "Clone Content" link wasn't displayed on my node preview page because I had a custom built theme that only output certain fields for each node. Try switching to another theme and see I that solves your problem.
The reason why the link didn't appear was because I had removed the following code from the page.tpl.php
if ($action_links): ?>
<ul class="action-links"><?php print render($action_links); ?></ul>
<?php endif; ?>
My suggestion is to check your page.tpl.php file so that the above statement really exists. If it doesn't simply copy the code into your template.
note remember to empty your cache after doing so.