iOS device location (outside of MDM Lost Mode) is provided by the SimpleMDM mobile app. Because of this, location data is sent to SimpleMDM through a different channel than MDM communications. Sometimes, a device will continue to communicate via MDM but will no longer provide location updates.
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.