My origin server has a valid certificate, but Cloudflare Universal SSL still isn't working. What should I check?

Double-check the certificate chain on your origin server. Ensure all intermediate certificates are included. Also, verify that your origin server's hostname matches the hostname used in the certificate. Firewall rules on your origin server might be blocking Cloudflare's connection attempts; temporarily disable them for testing. Review Cloudflare's SSL/TLS settings, particularly the 'Origin Certificate' section.