Common reason of failure of ReCaptcha token submission:
- token timed out - each token is valid during 120 seconds after passing the challenge, so you have at least 100-110 seconds after getting it from the API to submit it;
- website uses callback for ReCaptcha and you don't call it correctly;
- website uses some kind of additional protection and you have to find a way to override it.
Also starting from July 2018 there are cases when tokens are declined by ReCaptcha API. You can find more info
in our blog.