Sleep tracking Venu 3S - I primarily bought a smartwatch for sleep tracking, so far has missed 2 nights out of 4. First smartwatch - any tips or I have I bought a lemon?

I primarily bought a smartwatch for sleep tracking, so far has missed 2 nights out of 4. Is my first smartwatch, I would be glad of any tips - or I have I bought a lemon?

A couple extra points

- the 2 unrecorded nights, I was still up and about for approx 20mins AFTER the bedtime set in the sleep schedule. The 2 recorded nights that worked, I was in bed going to sleep at the set time. For a supposed "smart" watch of this $, I assumed that it would handle this sort of thing, but am wondering now if it did affect the sleep recording.

- battery was 15% when I woke up this morning - would this have stopped sleep tracking last night?