I've found that occasionally there can be a slight delay between when the subdomain DNS updates and the SSL certificate starts working. That said, I see you asked this a while ago, so if you're still having problems I'd reach out to ConvertKit's support team at help@convertkit.com48 (or with the message bubble in the lower right corner) and see if there might be something going on with your account.

Aaron HockleyCommunity Guru
Photographer, Writer, Consultant