If a staff member does not appear as an option during online booking, it's due to 1 of 2 reasons:
The staff member is not assigned to the target service.
To remedy this, assign the staff member to the service (go to the "Services" tab > Assigned Personnel)
The staff member's online booking setting is disabled.
To remedy this, enable the staff member's "Enable online booking" setting (go to the "Personnel" tab > Booking rules).