When visitors submit your contact form, the submission is rejected with the message "The reCAPTCHA was invalid. Go back and try it again."
Why this happens. This is a site configuration issue: two different versions of Google reCAPTCHA are active on the same page at the same time. The site-wide spam protection installed at launch conflicts with an older reCAPTCHA still attached to the form itself. The two interfere with each other, so the check can never pass and every submission is rejected.
What to do. Contact support to have the duplicate protection removed. When you report it, include which page the form is on and whether your site has any other forms, as this problem usually affects every form on the site.
Nothing needs to be changed in the form fields, and your visitors do not need to do anything differently.