Invalid captcha appears even when i enter correct code

wpDiscuz - Wordpress Comments was working great before i installed antispam plugin. After it, whenever someone tries to comment on my post, it says invalid captcha code, even when they enter correct code.

After experiencing this, i deleted antispam plugin, but still i experiencing same problem. Don't know what to do. Please help me

My website is:- www.highestproteinfoods.com