Biometric fingerprint log in on Dell Latitude E5540 with windows 8.1

I have a problem.  If I switch on my Dell Latitude E5540 laptop running windows 8.1 and I log in using the biometric fingerprint log in, I manage to log in successfully.  However while using the laptop and after some time of non usage the laptop reverts to the login, and if I try to log in again with the biometric fingerprint, this does not work and I can only login with my password.  Even the Biometric fingerprint icon does not then show in the login screen.  It seems that some program that I am using after the first login or a program which starts automatically in the startup programs, is apparently blocking this feature.  How can I find out which program is doing this.  If I switch off power and switch on again, thee b. fingerprint reader works only for the first time again.  Please help.