This can be due to several reasons, the most common being a transmission problem over the wireless carrier network. It could also be that the phone has been turned off, or depending on the device, the user has been on an extended phone call. Try calling the device to verify it is in network coverage.
Stopping tracking could also be due to the tracking settings. Based on the default tracking settings, a reasonable delay of up to 15 min can be expected. If there is a larger gap, check the tracking settings and times for the device.