using the_content hook on a wordpress plugin

I’m trying to add a div with certain content in every page of a wordpress.
For that I am trying to create a plugin that adds this div in a filter.
I’ve created the plugin, the function that modifies the content and the call to that filter with the hook “the_content”:

function add_content_filter($content){

      $original_content = $content ; // preserve the original ...
      $add_pre_content =  '<p> This will be added before the content..</p> ' ;
      $add_sur_content =  '<p> This will be added after the content..</p> ' ;
      $content = $add_pre_content  . $original_content  . $add_sur_content ;

      // Returns the content.
      return $content;
  }

add_filter('the_content', 'add_content_filter');

For some reason, even with the plugin activated the content is not modified by the plugin. I used the code from:

How do wordpress plugins add content?

as a refference. Do I need to do something else in order to get the code working?