Only events that the synced calendar organises are synced
When you create a sync connection to a calendar, it will only sync events that are related to that calendar via a dimension field lookup. If you add reservations to the calendar via a dimension junction field lookup (e.g. you can relate multiple records to the reservation of this type). The sync connection will not sync these.
In outlook, when you have an organizer of an event and then other calendars (which can be persons/rooms) that are invited. Booker25 will only sync events that you are the organizer of. Not events that you are invited to.
However, if you for example organize an event on your calendar and it is synced to Booker25 when you invite a location to your event and that location is also synced to Booker25. Even though the location is not the organizer it will appear in Booker25. Since it will be synced to your staff calendar and the resource field will be filled with that location since it does have a sync connection.
Sync Field Priority can cause fields to not sync correctly
When syncing into multiple external fields the field that will be the organiser of the events should have the highest priority. If it does not have the highest priority, any fields that have higher priority than the organiser field will be left empty.