I've also noticed this problem and if I'm not wrong somebody else also pointed to it the other day and this is what I do so I could see the recaptcha :

I did try reloading the page after using a VPN in order to change the proxy ( make sure you will only reload the first page where your going to register or log in and then turn off the VPN again, after that you may type your log in emails and password and click log in ) Note: using a proxy on entering inside the mining game is against the rules so only change proxy until the landing page or the first page where in, you have to type your login credentials and after that turn of the VPN before logging in.

This worked for me in order to see the recaptcha, maybe aliser could also check this matter to see why it's causing this problem to happen for some