jQuery ajax request breaks drupal behaviours
I presume you need to add that code with in document ready. Some thing like give

// Code that uses jQuery's $ can follow here.

(function($) {
Drupal.behaviors.mymodule = {
    attach : function(context, settings) {
      jQuery(document).ready(function($) {
         var txtFullname = jQuery('input[id^=my_dynamic_id]');

         // do ajax request
         jQuery.ajax({
            type : "POST",
            url : 'http://example.com/retrieve_information',
            data : {
                'order' : settings.order_id
            },
            dataType : "json",
            success : function(data) {
                txtFullname.val(data.fullname);
            }
        });
      });
    }
};
}(jQuery));