I have noticed, especially if I've recently washed my hands, my fingerprint will fail to unlock my phone. Do you have the same problem?
Rub that clean finger along the side of your nose, where it meets your face. Just a quick swipe will do.
Now, try unlocking your phone with your fingerprint again.
It will work now because you've transferred the oil from your face to your finger, allowing it to be recognized.
Completely clean fingers leave no fingerprint. Think about this, especially during flu season, and with coronavirus speeding around the world: your phone won't unlock unless your fingers are oily or dirty.
Clean your phone regularly.
If your fingerprint sensor fails to work a lot, and you wash your hands a lot, now you probably know why.