Bookings calendar admin has turned on Events on personal calendar affect availability.
It looks like only the main calendar (called "calendar") of a user's personal Office 365 can affect availability in the Bookings calendar as you cannot select any other calendars to share with your organization (Office 365 / Calendar / Settings / View all Outlook Settings / Calendars / Shared Calendars / Select a Calendar). Other calendars the user has added to "My Calendars" don't show - not even those added as 365 calendars and not from external sources.
@Mary-Lou have you figured out how to sync google calendars with MS Bookings app? This feature is pretty basic in Calendly - syncing several calendars into the master availability - why can't I see it in MS Booking app?