Hi
Apple advises that, even under ideal conditions, Apple Watch may not be able to record a reliable heart rate reading every time for everybody.
If the following setting was not enabled, then it may help to toggle it on and then off again whilst your iPhone and Apple Watch are connected via Bluetooth:
The following suggestions otherwise may help to improve the accuracy of readings:
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:
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 any 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 continue to 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 software issue or 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:
