If synchronization is not working, it means the device is powered off or the child has turned off mobile data.

Synchronization will also fail if you are on a metered data plan and have exhausted your monthly data allowance. This is because the synchronization command cannot be delivered to the device if it is not connected to the internet.

Just because you have recently spoken to your child on the phone does not mean the device is connected to the internet. Cellular voice networks and internet data networks are entirely separate.

When there is no internet connection, not only synchronization but all Mobile Fence features such as location tracking, rule changes, and timeline updates will fail to process in real time.