When I had tried to use
cptch_display_captcha_custom() on my own form, I did not succeed until I added also this line to my FORM:
<input type="hidden" name="cntctfrm_contact_action" value="TRUE" />
, because there is this check in
if ( isset( $_REQUEST['cntctfrm_contact_action'] ) ... ...
I think this is a bug, and that this hidden field should be automatically added while generating the INPUT by
cptch_display_captcha_custom() - not to have to add it manually...
PS: how to use BSW CAPTCHA on custom form see here: http://bestwebsoft.com/plugin/captcha-plugin/#faq