Does it prompt for fingerprint if you enter an empty password? If not, I have done a little looking around and found a source that states the following :
Add the following two lines to /etc/pam.d/kde
:
auth sufficient pam_unix.so try_first_pass likeauth nullok
auth sufficient pam_fprintd.so
This configuration allows you to first press enter on the empty password field to be prompted for the fingerprint