Hi
If you are not already doing so, then you might wish to use these settings:
- On your iPhone, in the Watch app, go to: My Watch (tab) > Passcode.
- Turn on Wrist Detection.
- Turn on Passcode.
- Enable Unlock with iPhone.
After putting it on your wrist, your Apple Watch will then unlock after whatever you do first: either unlocking your watch manually (by entering the passcode) or unlocking your iPhone. It will then remain unlocked (including when the screen is asleep) until you remove it from your wrist. When you next remove your Apple Watch from your wrist, it will lock automatically:
Turn on, wake, and unlock
Apple advises that, even under ideal conditions, Apple Watch may not be able to record a reliable heart rate reading every time for everybody.
To avoid your Apple Watch locking unexpectedly during general daily wear - and also for the best performance from the heart rate sensor during workouts - ensure that you wear your Apple Watch snugly (but not too tightly) on the top of your wrist. Apple suggests that you consider tightening the band before starting a workout and loosening it again afterwards:
Wearing your Apple Watch - Apple Support
The sensor is likely to give better results for workouts that involve rhythmic (eg running) rather than irregular (eg boxing) movements.
Other issues that can impact sensor performance include skin perfusion and wrist tattoos. As well as varying by person, skin perfusion can be reduced when exercising in the cold to levels that prevent readings:
For workouts that involve flexing your wrists, it may help to move your watch a little higher up your arm, away from the flexing joint. It may also help to check that the back of your Apple Watch is clean (along with your wrist, which I'm sure is the case):
Cleaning your Apple Watch - Apple Support
If you experience issues with recording your heart rate during workouts, then, for more consistent readings, you may wish to consider pairing an external heart rate monitor / Bluetooth chest strap to your Apple Watch:
Use Bluetooth accessories with your Apple Watch - Apple Support
If you are concerned about a possible hardware defect with your Apple Watch, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: