Try this checklist;
Check your personal details are correct in your health app profile.
Make sure the watch band is snug but not too tight.
Don't wear the watch on top of any tattoos.(Very important)
Check that the 2 side buttons aren't being accidently pressed by your hand or a sleeve
On your iPhone, go to Settings, then tap on Privacy and then select Location Services
option and ensure that it is enabled.
On the Location Services section, tap on System Services and then toggle the Motion
Calibration & Distance option to enable it
Next, go back to the Home Screen of your iPhone and launch the Watch application.
Then,tap on My Watch option, then tap on Privacy and toggle the Heart Rate option to
enable it.
Go back to My Watch section, then tap on Passcode, and toggle the Wrist Detection
option to enable it.
Finally, go back to My Watch section, then tap on Workout, and make sure Power
Saving mode is disabled. If enabled, toggle the button to turn it off.