Wrist HR Sensor Turning Off When Resting

Hi guys, 

Picked up a Fenix 7 yesterday and every time I sit down and rest/sleep etc my HR disappears and I just get --. It's there all the time if I'm up and about or tracking a walk etc. Had a Forerunner before this for 3 years and have never had this issue. Annoying as I loose all my stress and sleep metrics.

Been through all the settings but can't seem to find much. Done a soft reset, hard reset and restart. Any help would be appreciated. 

Thanks

Haydn