When locking the screen from XFCE, I assume its calling light-locker and then lightdm is handling the actual locking mechanism. If woke immediately the fingerprint auth works as it normally would at login, however if you let it sit and timeout, it will not allow logging in by fingerprint no matter what you do, even if you hit cancel which usually fixes the issue if it gets snafu at login. This bug was reported on launchpad and seems to have existed for many years and has been confirmed by many users in many versions of Ubuntu, I can confirm this bug also exists in Debian Stretch. Its very annoying. Here is a link to the launchpad bug report:
and here is a version table for my fprintd, pam packages, and lightdm packages:
I am just using a stock Debian Stretch, no custom configuration or 3rd party packages. The launchpad bug seems to suggest this was working in Ubuntu 13.10 I cannot confirm if this had worked in earlier Debian releases as I only just got a machine with a fingerprint reader this year. It does seem however from seeing the same issue in Ubuntu and confirmed for multiple versions, that this is not just an isolated issue.
It should be easily reproducible.. just lock the screen, take 5 min, go have at whatever your vice is.. a coffee, w/e, come back.. the finger print reader will light up but there will be no indication of it saying "swipe" on lightdm nor will it make the password box disappear. Dropping to tty1 and trying to restart fprintd had no effect, but restarting lightdm makes it work, however you lost your logged in session of course.
Expected behavior is of course for the fingerprint to work like it does for everything else, su, sudo, polkit, lightdm login, tty logins, etc.. and like it does for the lock screen if you unlock it immediately after locking. Only if it times out does it fail to work.