domain does not resolve, but machine does

[Frank Revi]

Inexperienced user here.  I am running a primary DNS server for, say,
domain.com.  The dns server is on machine.domain.com.  A whois search on
domain.com returns machine.domain.com plus the two backup dns servers at
the ISP.

nslookup domain.com fails; nslookup machine.domain.com works.

How can I get domain.com to properly resolve?  Thanks in advance...

-Frank