First, check to be sure that, for example, if your sub-domain is: abcd.domain.com - Check whether your sub-domain is being redirected or not. This is found under Subdomains or Redirects in cPanel.
If it is, be sure that it is NOT being redirected to domain.com/xyz, this will cause the sub-domain not to load.
If this isn't the case please submit a ticket to our support department.