A Metro station manager once chastised me for touching any part of the phone to the sensor at all (when mine accidentally double-tapped and wouldn't let me in because it thought I had already entered). He said you're supposed to just hold it above and I was doing it wrong. He was not particularly friendly about it, but I guess he gets lots of questions about this exact problem all day.
So, I guess try just hovering it? I usually still tap the end of it, though.