New reCaptcha implementation just loops

I have successfully implemented reCaptcha v2 on a number of sites but on my latest site after clicking in the I’m not a robot box it just spins and does not verify.

I copied the RecaptchaPlugin folder and RecaptchaPlugin.php from another working site so I don’t think the problem lies there. I have migrated my keys to Google Cloud, as suggested by Google, and the old keys still work. I have obtained new site and secret keys for my new site and installed them in the settings. I suspect that there is an extra step needed with Google but I cannot work out what it is. Any suggestions please?