reCAPTCHA was reported blocked in China in January 2016. I believe this is because parts of it are hosted under https://www.google.com/recaptcha/, and since SSL encrypts the URL being requested, this is indistinguishable from any other request to www.google.com from the Great Firewall's perspective
You can go through the discussion in [0] for more information. There are several methods/hacks described in the thread to make captcha work, please go through them and see what would work for you.