Someone's location is not updating

The usual causes, in the order worth checking.

A position that has frozen is the most common thing people ask about. It is almost always one of these, roughly in order of likelihood.

They are not moving

Circl reports when someone moves. A position that has not changed for an hour often means a person who has not changed position for an hour.

Their permission is not Always

With "While using the app", their position only updates while Circl is open on screen. See Battery and location.

They paused sharing

Pausing leaves the last known position on the map rather than removing them, so it looks identical to a phone that has stopped reporting. See Pausing location sharing.

Low power mode

Most phones cut background activity hard when the battery is low. A low battery alert followed by a frozen position is usually this.

No signal, or a poor fix

Underground, inside a thick building, or out of coverage. Circl discards readings too imprecise to be trusted rather than showing a position that is wrong by a mile, so the position holds at the last good one.

What will not fix it

Reinstalling. The position comes from their phone, so nothing you do on yours changes it.