Thanks to undertrained tech support staff, a common feature at too many registrars, the most common cause of custom domain problems involves failure to use "CNAME" referral - or "CNAME" referral targeting the wrong destination. Both use of forwarding, and "CNAME" referral to the domain root, is seen too often.
www.mydomain.com. 1800 IN A 18.104.22.168or maybe
www.mydomain.com. 1800 IN CNAME mydomain.comPlease, don't use either of the above address models!
Unfortunately, even with the blog properly published to the "www" alias - and with the "www" alias using "CNAME" referral, targeting "ghs.google.com" - we still see problems. With neither of the above mistakes made.