Custom Contact Forms Captcha problem

Hello,
I am using CCF and it has issue. sometimes it make the form submitted correctly and sometimes even after putting right captacha. it says wrong captcha. I tired in other browsers too.Same issue.what should i do?
p.s – i tried deactivating n activating again but nothing.

http://ww.wp.xz.cn/extend/plugins/custom-contact-forms/