The solution for me was to change my strap/band from the supplied plastic thing (!) to a woven nylon one. With the plastic one, I had 3 nights without sleep tracking; with the woven nylon one, I've have full sleep tracking every night for weeks!
I think this is because the woven nylon strap/band is somewhat elastic and allows some stretch as my wrist expands/contracts with changing temperature. The supplied plastic strap/band is quite rigid and I was always adjusting it for comfort.
I got my woven nylon strap/band from the uk version of the online retailer named after a South American river!
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