Is the 'Enable Custom Domain' box checked in 'Website Settings & Design'? 

Did you enter your custom domain name in Step 2 in 'Website Settings & Design' and press 'Save'?

Are the DNS Settings configured correctly at your domain registrar? (You need to add 3 A-Records & 1 CNAME for 'www')

A-Records:
3.209.36.65
34.239.225.42
54.158.235.112

CNAME:
'www' (name)
site.simplecast.com (value/points to)

Delete any additional A-Records (you should only have 3 total). It is okay if you have additional CNAME Records or AAA Records, but you should only have 3 A-Records.

Remove any domain redirects on your custom domain name

Using CloudFlare? Click on the orange clouds to disable any security features. Press Disable & Save when prompted. 

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.

Click on the orange clouds to disable security features

Click 'Disable'

Click Save