Problem with the user login and register plugin


I'm using this plugin to allow user login and register:

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:

However in these ones, There's no Captcha problem. User can register to the sites without an issue.

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.