Also, pay attention to the instructions when registering your finger - you need to move it around a bit for each of the four or five captures that it does on each finger.
The fingerprint reader is SO much better on this phone than it EVER was on my Galaxy S5. Not only is the position more natural, but it's a heck of a lot more accurate. As in "all the time". Having to swipe my finger/thumb down the home button was a PITA. Now, I can tap my index finger in ANY direction (my phone is sideways or upside down) and it instantly unlocks.