Oh! Sorry, I forgot to post the solution, yeah ^_^. For sure I will share it with you. But it is not a software problem.
The problem is that it is difficult to validate the fingers. My fingerprint reader is Authentec. The problem was that fprint recognized it 1 out of 20 times, and it was driving me mad.
Now I have improved it to 1 out of 4-5 times. The thing is that when u scan your finger you have to try to swipe it in the same angle, with the same speed, and applying the same pressure as you did when you enrolled it. It is a bit annoying, but I imagine that with practice I will be able to succeed validating it everytime.
Thats it =)