Hi
Apple Watch takes background heart rate measurements automatically only when you are still. The timing of such readings will therefore vary depending on your activity level, as explained in the following support article.
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Heart rate is automatically recorded continuously during workouts when recording them via the built-in Workout app (and third-party fitness tracking apps that support the feature):
Work out with your Apple Watch - Apple Support
Apple advises that, even under ideal conditions, Apple Watch may not be able to record a reliable heart rate reading every time for everybody.
For the best performance from the heart rate sensor during workouts, ensure that you wear your 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 - be sure to also record a workout via the Workout app, as detailed above:
Wearing your Apple Watch - Apple Support
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.
Also 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 another Genius Bar reservation or visit an Apple Authorised Service Provider for assistance:
