Apple Watch 5 - heart rate

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, as you have indicated that you are already doing. Apple suggests that you consider tightening the band before starting a workout and loosening it again afterwards:

You might try a reset by pressing in the crown and side button until you see the apple logo, then release both.