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.
For the best performance from the heart rate sensor during workouts, Apple suggests that you consider tightening your watch band beforehand and loosening it again afterwards.
The sensor is also 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 - which can vary from person to person - and permanent or temporary skin changes, such as some wrist tattoos.
If you continue to experience issues, for more consistent readings, you may wish to consider pairing an external heart rate monitor / Bluetooth chest strap to your watch.
More information:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Use the Workout app on your Apple Watch - Apple Support
Use Bluetooth accessories with your Apple Watch - Apple Support