Most issues with location tracking stem from permission settings within iOS. The easiest way to determine why a device is not providing location information is to open the SimpleMDM app on the device itself. The SimpleMDM app runs a diagnostics check whenever it is opened and will present helpful information on how to resolve any issues it finds.
Since a user can disable location tracking or background execution, and Apple does not provide a way of blocking this, there is no way at this time to force location tracking to occur in iOS during normal operation. As a result, location tracking is provided on a best effort basis. Please note that location tracking is still available when a supervised device has MDM lost mode enabled, as this function does not require the SimpleMDM mobile app.