I had this issue for over 5 days but it was connected to tattoos problem. Unfortunately when moved to other wrist problem still existed but now randomly - I can see sleep data for smal naps and last night, no sleep data for previous days even when rebooted.
Not once has it successfully recorded a full night's sleep! I've updated, restart it, reset it, had it tight, had it louse.. nothing.
It recorded a nap I had just fine... what the hell is wrong with this..