I have found the accuracy of sensors really does depend on the factory calibration, and the bad ones seem to come in batches (eg if I collect 4 from the pharmacy, they are often from the same batch, and the each batch seems to have similar performance). They all tend to overestimate highs and lows, and will show the biggest difference vs finger sticks when blood glucose levels are changing rapidly (after eating or exercise).
a good time to check the health of your sensor is when you’ve had steady reading for a little while, such as first thing in the morning, or before meals. If the readings at these moments are close enough, then you’ve got a good sensor.
they will never be spot on, but are supposed to be within 10% either way of a finger stick when glucose levels are steady.
hope that helps to set the correct expectations for what ‘working well’ looks like.
In my experience, about 1 in 5 sensors fail or are way off and need to be replaced, which is acceptable to me