motion activated towels

Alright, maybe someone needs to enlighten me on how these things work, because I cannot for the life of me figure it out. I’m talking about the motion activated towel dispensers in public bathrooms. Every time I wave my hand in front of it, sometimes it works, sometimes it doesn’t, and when it does work, only maybe 3 inches of towel come out. I need about a foot of paper towel to dry my hands, and so I keep waving my hands in front of the sensor, and it never seems to extend the towel. Why is that?