So I have this issue also. For me it worked the very first night I wore it, and then it has worked intermittently, as can be seen in the screen shot. I have now got 5 days of readings, but they are spread across over a month now, and nothing for the last 2 weeks! I have all the settings switched on as per recommendations. .
I am currently in the process of getting support from Apple. However this isn't going well...! We have been around the same routine several times, whereby they switch on enhanced logging on my phone, but for some reason, the log files don't seem to be sent (apparently this support process is fairly new!)
But my thinking is that as it HAS worked a number of times in the past, then the issue is most probably some variable relating to me/my sleep process/environment...? But I can't think what I did differently on those 5 times when it did actually work!
I've tried adjusting the strap to various levels of tightness.
Obviously my exact sleep times change, but engineering it around the set sleep schedule is not easy to test any connection relating to the schedule. But I've gone to sleep before and after the exact start time of the schedule and woken before/after it too.
I'm a bit reluctant to fully reset the watch, especially if it means not using the backup too.
But it is also frustrating that the Apple support process is so flaky - I'm currently doing the enhanced logging again now, so hopefully for this 4th or 5th time they will get a usable logging file.