Apple Watch sometimes records heart rate sometimes doesn’t

My Apple Watch always records my workout but sometimes (like today) the heart rate result is blank. I am certain all the settings are correct because some days it works but, just to humour it, I have just checked every setting, restarted the watch, restarted the phone and re-paired them, as suggested in all the posts about this. The heart rate for today’s walk is still blank. I need this to work because I have subscription payments for my watch dependent on points earned for workouts through my health insurer. What else can I try?