Filters don’t work on syndicated content

I’m having trouble applying filters to syndicated content (using FeedWordPress).

The following test plugin works for content I manually post, but not for syndicated content:

<?php
/*
Plugin Name: A Test plugin
Description: A test plugin
*/
function test_function($content=”)
{
return “TESTING”.$content;
}
add_filter(‘the_content’, ‘test_function’);
?>

What is different about syndicated posts that this doesn’t work for them? I tried messing around with the wp_posts table post_author, excerpt, and guid fields to see if making those like my own posts would fix it, but that didn’t seem to work.