I am attempting to append for this code:

<table class="stores" border="1" cellspacing="10">
    <tr>Stores:</tr>
</table>

This code, which I wish to work, does not:

$('.stores').append('<tr>Test</tr>');

That one works:

$('.stores').append('<p>Test</p>');

Are you able to say why?

P.S. Unsure it matters but I am working under WordPress.

You cannot have text between tr-tags. Do this:

$('.stores').append('<tr><td>Test</td></tr>');

That isn't valid HTML. You are missing the <td>

Works fine whenever you alter the markup : http://jsfiddle.net/jomanlk/XHvVk/

$('.stores').append('<tr><td>Test</td></tr>');