Cause: Sendiio uses a JavaScript-based form POST, and something is preventing the redirect.
Fix:
Make sure JavaScript is not being blocked by the visitor's browser or an ad blocker.
Check that your List ID and Form ID are correct in the Sendiio card on the Autoresponders tab.
Open your browser's Developer Console (press F12) on the funnel page and look for JavaScript errors.
If you see CORS errors, this may be a Sendiio-side issue. Contact Sendiio support to verify your form endpoint is accessible.