Captcha Promblem

Hi there, I got a problem on one client’s website.
We’ve added captcha on a contact form, and at first time charging the page, when we validate the form an error occurs “Entred code (captcha) is incorrect” at this moment change the captcha image and the second captcha entry is correct. Can’t find what is the cause of this problem, but do the same thing every time. Please help

https://www.ads-software.com/plugins/contact-form-7/