Your attendee may be having some issues with their access and you have to check if they have the correct ticket.
Head to Reporting > Attendees.
Click the "..." next to the attendee record.
Select Manage Tickets
This view provides a comprehensive overview of all tickets purchased by the attendee, along with any associated restrictions, such as an expiry date. Additionally, you have the flexibility to manually check in attendees for in-person events, remove a ticket, transfer a ticket, and assign a ticket as needed.