Heart rate monitoring switching off during exercise - Why?

Hi, I have checked the historic posts on this, and I have ensured that:-

In iPhone Settings > Privacy > Motion & Fitness, both Fitness Tracking and Health are both ON

In iPhone’s Watch app : Workout Power Saving Mode is Off; In Privacy both Heart Rate and Fitness Tracking are both On.

I can measure my Heart Rate, so that is functioning, but when I go for a walk, Heart Rate tracking does not take place. The Cardiogram app shows a great big blank zone where the exercise was taking place.

This seems bizarre, as thats exactly when you need it most!

If I cant sort this out the watch is going back so all help very much appreciated.