As Been A User/Being A User Of Samsung Phones With Fingerprint Scanners I Have Faced Many Problems (Some Which I Have Fixed) If You Are Struggling To Scan Your Finger This Could Be Sign Of Damage I Had This Problem With My Previous Phone (Samsung Galaxy S6) And I Couldn't Scan My Finger. Also With My New Phone (Samsung Galaxy S9) It Has Been Dirty Many Times Either From Fingerprints,Dust Or Dirt And It Has Unfortunately Failed To Recognise My Finger But After A Quick Clean I Got It Restored So If You Are Having A Similar Problem Do Check For Damage Or Dirt/Fingerprints/Dust
A couple of tips I picked up when setting up the finger prints are.
When setting up take x3 scans with each thumb and then rotate the phone around when taking further prints of each thumb so your phone can read both thumbs from the same finger print scan. Scan your prints as you would normally hold the phone.
Also using two fingers when setting up one fingerprint works pretty well.
Go into Settings . Biometrics and Security. Biometrics Preferences . Turn Off Screen Transition Effect.
Then go into Settings and search for Optomise Battery Usage. Tap in Optimise Battey Usage tap to show All and scroll down to com.samsung.android.biom and toggle Off
For me this improved the fps on my Samsung phone's.
I also have dry skin on my fingers and so find it sometimes beneficial to moisten my fingertip by breathing on it before unlocking the phone.