.js files not being included; form still submits w/o ajax even when inclusion is forced

This is on WIMP, D6.12, Ajax6.14.

So far, I have:

1. Installed and enabled Ajax and Ajax_UI modules.
2. Gone to Ajax Settings page and enabled Content Type: Page.
3. Gone to node/add/page and submitted the form without a title.

Result:
Form submits as usual with usual Drupal error; no alert box.

Having read this other similar issue (http://drupal.org/node/436432), I checked to see if ajax.js was being included in $scripts. It was not. So, I moved the drupal_add_js calls from ajax_page_preprocess to ajax_form_alter. That caused them to be included, but the form behavior remained the same.

I feel like I'm probably missing some blindingly obvious step in the configuration, but I've read and re-read the docs. Help, please?