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@Derms: The phone should track your steps using it's sensors, however if you're tracking your route using the GPS/Location feature then this may be the reason why some of the steps aren't being recognised. Can you try heading to Settings > Connections > Wi-Fi > Tap the 3 dots in the top right > Advanced > Switch To Mobile Data, and toggle this option on to see if this helps? If this feature is already active, toggle it off and enable mobile data manually before you go walking, as you'll be out of range of your router shortly after leaving the house.