Why do some contacts see 'No tickets available or tickets have expired' during registration?

Some contacts trying to register for my event see the message 'No tickets available or tickets have expired! Or tickets are only for registered users with specific roles.' I can register just fine. Why are others unable to register?

This issue typically occurs when ticket types are restricted to specific user roles. If the ticket type setting "Ticket type visible to selected usertypes only?" is enabled and certain roles are selected (e.g., reviewers), only users with those roles can see and register for those tickets. 

To resolve this, go to each ticket type in the event administration panel, locate the setting "Ticket type visible to selected usertypes only?", and uncheck all specific user roles. Save the changes to make the tickets visible to all registrants.