Does this mean it used to work OK and it's suddenly stopped? Do the other monitoring features (heart rate, stand time) work?
1. Restart the watch - hold the side button until a menu appears, choose Power Off, wait a few seconds and then power back on
If that doesn't work:
2. Reset the watch - hold both the digital crown and the side button for around 10 seconds - until the Apple logo appears
If it still doesn't work,