Appointments+ unable to book multiple clients to a session

The school offers tours for up to 3 families per tour, however currently only 1 visitor can book the school tour service. After that it is shown as unavailable to subsequent visitors.

I've set up the service, school tour, with the capacity set to 0, 1, 3, and 10.

I've set up 4 service provides, 2 real user and 2 dummy service providers and assigned them all to provide the 'school tour' service.

I've cleared the cache. Any advice on how to get this working?