My Touch ID on my iPhone 7 has been working fine for years, set up with about 3 different fingers.
Suddenly, a few weeks ago, Touch ID stopped working.
It doesn't recognize that I'm putting a finger on it, whether just trying to unlock the phone, or set up new fingers.
However, it DOES sense my finger being placed on it for using the home button for purposes OTHER than Touch ID.
My iOS is 11.3, and I am not going to update the iOS soon, so no need for you to suggest that as a possible solution.
Also, in the last couple/few weeks, 2 new problems I've noticed in my iPhone Control Center:
* it no longer presents 4 brightness levels to choose from, when holding finger on flashlight button (however, it DOES still turn flashlight on and off)
* it no longer presents Screen Recording settings, when holding finger on Screen Recording button (however, it DOES still initiate Screen Recording)