Bummer. Could be defective. Try this:
Go into the fingerprint registration screen, and repeatedly touch the home button
as if training or unlocking. As a registered finger is recognized, the entry for
that finger will briefly low-light. It's a sanity check I guess. If a finger is being scanned
in the same position as it was registered, and assuming the finger is stationary
(Very Important) and the entry is not flashing, there may very well be a problem
with the sensor. BTW, I don't know for sure, but my gut feeking tells me
this action help in the system learning and getting better over time, FWIW.
I think on days I have too much coffee I find I have to try 2-3 times to unlock.
Either that, or I forget to position correctly, look at where I hit the home
button, reposition and it it's fine.
Also, if you position your finger to unlock without depressing home, but rather
hit the sleep/wake button without moving the finger on the home button,
it should unlock fine. It was tough at first to realize I was moving my finger,
ever so slightly, while trying to unlock. Given just having depressed the home
button, I guess it's to be expected.
HTH.