Question

Why can the fields I am aiming to sync from the UI not be configured in the connector setup?

Environment

Connector: Facebook Ad Insights

Answer

Check if the required field is a part of an action breakdown, then configure the connector with that particular action breakdown selected. 

For example, the action breakdown action_type ​  contains:
app_custom_event.fb_mobile_complete_registration - Mobile App Registrations 
offsite_conversion.fb_pixel_complete_registration - Completed Registration

Cause

Some fields are synced as part of action breakdowns.