There's a lot of moving pieces involved here... you'll probably get the best resolution by working directly with ConvertKit's support folks. You can reach out to ConvertKit's support team at help@convertkit.com23 (or with the message bubble in the lower right corner).
If you want to try to troubleshoot on your own, we'd need to know the domain name, the subdomain name, your current DNS records, the domain/DNS settings from ConvertKit, and the custom domain configuration for each of the landing pages in question...