Someone has to tell others that your DNS is the secondary of a given domain. If it's not done, then your DNS is possibly working for nothing: it won't receive any query for that domain.
This job must be done in the primary specification (see DNS advertising). So, if your DNS is a secondary, it is not your job :-) The primary must explicitly name the secondaries. Further, it has to register this information in the DNS of its parent domain. This is often forgotten.