I know it might sound strange, but you should try to assign fingerprints from other fingers as well because even a simple irregularity on the main one will provide a wrong reading. Try removing the fingerprints and choose other fingers, then try to log in again. If you experience the same behavior, then it's a problem with the software or with the hardware device that reads the fingerprints.
I would recommend a re-installation of the driver or the software, then try again.