reCAPTCHA v3 is based on a user score. If users are being rejected based on the captcha (and assuming the API keys being used are for v3), they are being scored too low.
The plugin currently uses Google’s default of 0.5 as a necessary score to clear the captcha. (See:
https://developers.google.com/recaptcha/docs/v3)
If you would like to lower the score, you may proceed in using this snippet utilizing the filter
wpum_recaptcha_score_threshold
Get Started with the Best WordPress Membership Plugin Today
Connect, Manage and Build your Membership Site