Domaing Mapping NOT redirecting subdomains to custom domains

Subdomains: subdomainname.mywebsite.com is not redirecting to custom domain: subdomainname.com….. Please check:

<p>Mywebsitelink </p>

<p>A subdomain example</p>

<p>A domain for the subsite</p>

Please advice, this is CRITICAL!

thank you.