[Plugin: SI CAPTCHA Anti-Spam] not working!

First check this: make sure the only other security plugins you have are Akismet or WP-spamFree. Akismet and WP-spamFree are the only other anti-spam plugins approved for use with SI CAPTCHA Anti-Spam, others can simply break the CAPTCHA validation so that the CAPTCHA is never checked. If another security plugin is combined(not Akismet or WP-spamFree), the captcha may not work. Be sure to always test the CAPTCHA after installing any new plugins.