Here's what I received from someone who would like to sign up to our forum: 

  Quote

 I would like to join your forum. However, when I fill in my details in the "Sign-up" window, there is nothing to enter in the field entitled "Security Check". Please can you help?

Any idea what is going on, and what I should do? I've asked them what browser they're using and if they could send a screenshot. 

The browser used is Safari.

They sent me the following screenshot (I blanked out the password but you can see the hcaptcha is missing): 

Ask them if they have an ad blocker, and if so to try with it switched off.

Ok thanks!

Ok so the person said that they're not using an ad blocker, they're seeing all the ads, and they're even seeing the captcha when clicking "Contact Us" at the bottom of the forum, but not seeing the captcha in the sign up form. 

Hard to investigate without access to the device. 

But I would try the regular (not pop-up) form and, if necessary, reloading that page. It available at yourdomain.com/register/

 

I would also have them test with a different browser and then a different device. For example if it happens only on Safari but not Chrome, it’s something in their browser. 

If it happens on all browsers on the device, but works from a different device (such as a cell phone), it’s something installed on the device (malware, some sort of VPN or something else local to the machine).