HRM not recorded at night

Hi, I could use some swarm intelligence.
I've purchased a Bangle.js2 two weeks ago. During some nights no HRM or movement data were recorded. I already checked the designated app on the watch and could see a large gap beginning when I went to bed and ending when I got up in the morning. I already checked for the following:

1. wrist band tightly worn, a finger fits between band and skin but moving the finger is hard
   
2. watch not covered by pillow or other pressing material
   
3. even at nights where I read in bed but are still awake, no HR is recorded.
   
   

Before you might suspect that I didn't enable it, I have HRM's for the rest of the day.

Then, some other nights, I had records but they didn't show up in Gadgetbridge, all other HRM data earlier and later however are in the diagram. I should say, in the activity diagram because I don't have any data in the sleeping diagram.
Of these 2 weeks I own the watch only 3 nights show that I slept. The rest have no data at night.

Any thoughts on this?