There are two different pricing/service options with SPOT;
basic - which requires the user to press a button to transmit your location, and
tracking - which provides the 10 min auto transmit (for 24 hours) feature
The 45 SPOTs that were hired from USA are using the full tracking service.
The 10 SPOT's hired in New Zealand are using the full tracking service.
Some of the privately owned SPOTs are using the basic service. So these trackers will appear to be much more intermittent than the others.
Here's some background info from the SPOT website on the tracking service:
Once activated, SPOT acquires and sends your GPS coordinates to your SPOT account every 10 minutes. Anyone with access to your account information can log on and view your route, complete with virtual views provided by Google Maps™. To continue tracking on longer journeys, the SPOTcasting™ function must be re-activated every 24 hours.
Also; like any GPS device - they may have trouble communicating if the rider is under thick native bush.