Problem with the user login and register plugin

Hi,

I'm using this plugin to allow user login and register: http://bit.ly/1YY6lbX

My site users can no longer register to some of my sites because they are facing a issue with CAPTCHA verification. I tested this as well once all the details are correctly entered CAPTCHA verification always return as wrong CAPTCHA. It always says "CAPTCHA is not valid, please try again"

In These sites Captcha is not working:
http://bit.ly/1rGyDO4
http://bit.ly/1pQKIy7

However in these ones, There's no Captcha problem. User can register to the sites without an issue.
http://bit.ly/1SR6uMG
http://bit.ly/1SR5QM8

Can you please diagnose what's going on with those 2 sites? Support access is granted.
Its wired that Captcha works on some sites but not on others.

Thanks.