Cannot log into mapped site with admin account

Sites in my network that are subdomains of the main site all work as expected. :
networkdomain.com (main network)
subdomain.networkdomain.com

Sites in the network, but mapped to their live domain: for example---mappeddomain.com, logins are not getting in. mappeddomain.com is a mapped domain in the networkdomain.com network. The only user on mappeddomain.com is admin. This admin account is the super admin account of the network. Trying to access the dashboard of mappeddomain.com from within the networkdomain.com redirects to mappeddomain.com login prompt, but the admin account fails to log in.

I am sure there is a simple answer but I am stumped. Any help would be appreciated.