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I retrain it all the time. It doesn't help. If there's any skin oil or sweat on the sensor it doesn't work.

It also gets confused if there's any dead skin on my thumb, something that seems to happen pretty often (and I don't even play Nintendo anymore).

I've always just entered the same finger for every finger option, not really sure how much it helps however.

This is the problem I have. If there’s just a little sweat on my finger, it stops working.

Just for another piece of anecdata: mine used not to work if my finger was too dry... I resorted to everything up to and including licking the finger and retrying. Which, oddly, usually worked.