AHAH not triggered for second step in cascading logic

We are using the AHAH helper module for a page which involves the following requirements: the user selects from a list of review boards. Based on that selection, a list of research topics associated with the chosen board is presented. When the user selects one of the topics, checkboxes for published articles related to the chosen topic, and checkboxes for reviewers assigned to the chosen topic are presented.

The module is working correctly until the second selection (from the list of research topics) is made. We can tell from debugging that the AHAH callback is not being invoked when the selection from the second picklist changes.

Is this level of complexity not supported by the module (or by the #ahah mechanism in Drupal)?

I have posted the source code for a repro of the problem, boiled down to the minimum needed to reproduce the failure, at http://pastebin.com/YTHPRQP1 (I tried to post the .module file below but the reporting interface won't accept it; I'll attach the .info file for the repro). The repro is a modified version of demo module shipped with the AHAH Helper module.

Thanks!