I've added these lines to the form page:
<?php require_once("webassist/form_validations/wavt_scripts_php.php"); ?>
<?php require_once("webassist/form_validations/wavt_validatedform_php.php"); ?>
They already exist on the processing page.
When I leave the Captcha field blank, I get the validation to show up: "Attention! Please enter the CAPTCHA code."
When I enter the correct code the form submits and everything works as expected.
When I enter an incorrect captcha code, the form appears to process for a while but stays on the screen with no error displayed.
Thanks for your help!