I put my on chest as I am very active in exercise. Try on arm get knock off even put tape on. Use arm a lot in gym n swimming so sensor seem not good on arm. Try thigh on leg also great spot.
I scan using reader first then use phone to scan. The phone can’t even scan the sensor. It need a practice I’d professional to connect. Can’t even check on the sensor icon on phone it just freeze. No data show up. But other things on it can cluck.