When you use AHAH buttons to update (pieces of) a form, and the replaced markup (the data returned by the callback function) contains new AHAH buttons, the AHAH behaviors are not attached to these new buttons. This is caused because the ahah behavior uses Drupal.settings to loop through all AHAH buttons. While each AHAH button has a bunch of settings attached, using Drupal.settings is preferred over using $('.classname:not(..)').each(...., which is a more common way to attach behaviors.
If the AHAH does it's request, and the callback function is built, AHAH creates a new Drupal.settings object in the background, containing all (new) AHAH buttons.
How about rewriting the Drupal.settings object by default after each AHAH request?