To ensure the selected unit is always in the centre of the map you must Lock their device. You will notice an open lock image next to each device name in your devices list. Although you can plot multiple devices at any one time, only one can be followed and kept in the map centre.

The person to be tracked must have a check next to their name for this to work. If they do not have a check, click on the check box before clicking the Lock image. After this refresh the data points by either clicking the Refresh data points link in the bottom right of the page, or clicking on their name in the device list.