I suspect it's got something to do with skin oils or natural variation in skin moisture. I know that big humidity swings were guaranteed to ruin it for me. Air gets dry in the winter, touch unlock drops to maybe 20% success rate at best. Fix it by re-training, then in late Spring it does it again when the air gets wetter.

The ones on their laptops, however, have been reliable. Possibly there was a generational improvement in the equipment before it hit the laptops, and my problems were just because I had an early model on my phone.