Sleep tracking doesn't register all parameters


When I wake up in the morning sometimes I don’t get a HR reading through my Samsung watch, sometimes I do, and I don’t know what I’m doing wrong. Sometimes the reading of the sleep cycle appears naked then I rate its quality with the stars and then press the check symbol on the left upper corner. I wait a bit and then as I said before the heart rate appears.
Has it have something to do with keeping my hand covered with the sheets?
Also regarding sonar, sometimes I get breathing rates and sometimes I don’t even get the deep sleep readings within the sleep cycle.
Any thoughts on this?

Best regards,