I started having the same issue Monday, I think I figured mine out. I did a lot of pressure washing over the weekend and my hands have been a bit dry, it kept telling me to cover the entire sensor which I did over and over, so I thought the sensor had gone bad. I added a second finger under the settings, it sees that one just fine and unlocks every time now. Could be using my pressure washer trigger finger for so lomg has temporarily rubbed down my fingerprint, I'll use the other finger for a few days and see if the original one comes back.
I am having more and more problems with my Samsung 8 plus every time they do a software update. Which seems to be happening every other week. 1st the microphone says what it once to say unless I articulate every single syllable. Then my finger print scanner will load my same fingerprint 4 × 2 make sure it is every possible angle then all the sudden it doesn't work. I have wiped it off I've reset I've safe mode nothing is fixing this.