Captcha not working – User still manage to login without Entering anything

Hi thanks for your script…
I have just activated the plugin but its not working properly.
I see the Captacha enabled in the login area but I am able to login with my username and password without entering anything into the box.

Please let me know how i can solve this issue.
Thank you

http://wordpress.org/extend/plugins/alphaomega-captcha-anti-spam/