Could be a number of reasons.
Have you tried deleting and reregistering your finger prints.
If you've recently put on a new screen protector, it's recommended to redo your finger prints.
Can try Wiping the Partition Cashe on the phone. Can sometimes help
Also try turning off the always on display and see if that makes a difference. I remember there was a bug with this that affected the sensor. If so try seeing if there's an update for the app in the Galaxy Store
