I have never had a problem with the biometric scanners (1-5 uses per week for the last year), and only a very small percentage of guests seem to have trouble with the scanners (from my own personal observations at the turnstiles). The primary causes of problems seem to be
1) too many fingers,
2)Sun shining on reflector,
3) Wrong hand,
4) Arthritis, and
5) hardware problem.
If the sun is out and at a low angle of incidence (morning or afternoon), shield the reader with your free hand and see if that helps.
I also see people removing their fingers too soon quite often. Your fingers have to be still long enough for the scan to complete, and wiggling or a too-fast "in and out" will not always have positive effects.
Don't let these little technology problems get you too down... I get people every now and then who will stand in front of the stile, ticket in hand, glaring at the machine. When I tell them that they should put the ticket into the machine, they say, "What? There has to be a better system, this is the Computer Age!" God help us if computers are ever able to read someone's mind...