add_filter("the_content","…") hook not working

Hey,

I developed a payment plug-in for WooCommerce and it worked fine until recently but now one of the hooks it uses isn’t being called it seems.

The hook is:
add_filter('the_content', 'showMessage');

function showMessage($content) {
            return '<div class="box '.$this->msg['class'].'-box">'.$this->msg['message'].'</div>'.$content;
        }

I’ve also tried:
add_action('the_content', array(&$this, 'showMessage'));

with no luck. The whole plug-in file looks like this:

[Excessive code moderated. Please use a pastebin.]