Trigger a geofence vs enter a location

Hi all, 

Does anyone know the difference between 'Trigger a geofence" and "Enter a location" for Action-based delivery? I'm working on location-based push alerts and trying to understand why some people are receiving messages and others are not. It's complicated by location settings in our apps, but it appears "Enter a location" is the action that works. I haven't gotten "Trigger a geofence" to send. 

Thanks! 

Dustin