UMI FAIR'S FINGERPRINT WORKS ONLY SOMETIMES. HELP!

MaaDS

Hello. My name's  Alexandra and I have an UMI Fair. When I unpacked it, the fingerprint worked. But after a few lock/unlock testing the fingerprint, it works sometimes. I mean, sometimes when I put my finger on the fingerprint the mobile phone unlocks and sometimes doesn't happen anything. the same with the settings: sometimes when I try to add a new fingerprint works and sometimes not. *****I HAVE TO SAY THAT SOMETIMES WHEN I TRY TO ADD A NEW FINGERPRINT THE MOBILE PHONE DOESN'T DO ANYTHING (LIKE A LAG,WITH WHITE SCREEN) AND I HAVE TO PRESS THE HOME BUTTON AND TRY AGAIN. I THINK THAT IS NOT THE FINGERPRINT BECAUSE IT WORKS BUT I THINK THE SOFTWARE IS THE PROBLEM. TRY TO MAKE A NEW UPDATE TO THE SYSTEM (ROM) AND SOLVE THE FINGERPRINT PROBLEM PLEASE!!!!