SOLVED! - After posting this question I continued searching and found a previous support article on the plugin page. The response there from Formidable was to disable any cache plugins in case they're clashing - I was using Breeze by Cloudways. I disabled it and the captcha is now showing.

Maybe there's a way to get both working in harmony with some kind of setting somewhere (can Formidable suggest something to try?), but for now, it's working :)

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I have recently installed Formidable to add a contact form to my website with Captcha protection.

Initially I added ReCaptcha (tried V3 and V2) but I kept seeing the "ERROR for site owner: Invalid site key" message, so I tried moving to hCaptcha - but the same result appears.

In terms of troubleshooting so far, I have done the necessary setup both on the Google side for ReCAPTCHA (trying both V2 and V3 versions of the keys on the classic login) and the same for hCaptcha. I have checked the keys and checked for erroneous spaces etc. I have also purged Cloudflare and tried different devices/networks.

Here's something I've noticed - if I'm logged into WordPress the hCaptcha box appears properly when viewing my contact page. As soon as I log out (or use an incognito window), the error message re-appears.

Does this behaviour give any clues as to what is causing it to go wrong when logged out (i.e. most of the people who will be using this form)? I ask because I really don't want to start stripping everything back to try and identify a plugin/theme issue if there's a quick, known fix for this I can try first.

Thanks in advance.