Using CNAME to map domain to child site

Hi

I'm running WordPress multisite on a dedicated IP. I've installed the Domain Mapping plugin and followed the guidance to set it up correctly. I can successfully map a domain to a child site using the A record method but I'm unable to map a domain to a child site using the CNAME method.

I have child.mynetwork.com created as a child site and I have mynewdomain.com which I'd like to map to the child site. In the registrar's DNS settings for mynewdomain.com I have set the www CNAME record to point to child.mynetwork.com. In the child site's dashboard, under Tools > Domain Mapping, I have entered mynewdomain.com in the mapped domain field. However, when I click the 'Map domain' button, I get the red warning message to say that the domain name is unavailable to access.

However, checking through intodns.com, I can see that the records have propagated for mynewdomain.com and that www is pointing to child.mynetwork.com.

Testing a different child site with another name using the A record method is successful so I am at a loss to know why the CNAME method is failing.

Any help would be appreciated.