This may happen when you configure your domain name with the www prefix (www.mydomain.com) to point to your Simplecast site, but you only specify the bare domain (mydomain.com) in your Website Settings. Ensure the two URLs match in the Website Settings of your account.
Also, depending on your DNS provider's TTL (Time To Live), there may be a delay of up to 24 hours for the change. Most DNS providers will allow you to lower this TTL window.
If you still have questions, please don't hesitate to contact us through our live chat system.