Back green light not working after removing from charge

the heart rate monitor stops working on and off on my vivoactive 4s as soon as I remove it from charge. The green sensor light is shut down and does not switch on again. Only after a few hours.

watch is touching the skin correctly.

The back of the watch is clean.

Tried turning on and off several times.

Tried reset to default. Only when I reset the watch it works.