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I have tested your form in Google Chrome and Captcha is working properly. However I have recreated your problem in Mozilla Firefox. After inspecting your website I noticed that you are using script block (source code) for embedding your form on website. We recommend using of iFrame code. You can try to replace source code with iFrame to see if the Captcha will load. In this article you can find out how to do it.
Hope this will help. Let us know if you need further assistance.