Check the plugin settings, which are found under Plugins > Conditional CAPTCHA. The default behavior for “When a CAPTCHA is not completed correctly” is “Leave the comment in the spam queue”, so you may not notice a difference since the comments end up in the same place. Try changing to “Trash the comment”, and you should see most comments end up in Trash instead of the Spam folder. Once you’re satisfied it’s working, you might want to change it to “Delete the comment permanently”.
I also recommend using reCAPTCHA; it requires a signup to get a key set, but it’s free.
On one of my sites over the past 1.5 years:
279,248 spam comments have been blocked by Conditional CAPTCHA
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