Hi
The following steps may help:
Check that Wrist Detection is turned on and that, if passcode is enabled, you are unlocking your watch after placing it on your wrist
- On your iPhone, in the Watch app, go to: My Watch (tab) > General > Wrist Detection - turn on.
On your iPhone, go to: Settings > Privacy > Motion & Fitness - check that both Fitness Tracking and Health are enabled.
On your iPhone, in the Watch app, go to: My Watch > Privacy > Motion & Fitness - turn on Fitness Tracking and Heart Rate.
Check that you are wearing your watch sufficiently snugly on the top of your wrist (if it is worn too loosely, your watch may believe that it has been removed from your wrist).
Check that your personal information is accurate:
- On your iPhone, in the Watch app, go to: My Watch > Health > Edit - tap and adjust items, then tap on Done.
Calibrating your watch improves the accuracy of various activity-related estimations. Instructions for how to calibrate your watch are available via the support article linked below.
More information:
Use the Activity app on your Apple Watch - Apple Support
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
Use the Workout app on your Apple Watch - Apple Support