Maybe help to somebody: i use my index finger nail and triple tap.
I have trouble with double tap too. Seems like I always have to double tap twice to get it to work. Have tried a more firm tap in the past and that didn't seem to help for me.
It does not work very well. I thought implementing a double tap was easy, but it seems it is not (or Garmin has not yet figured out how to do it).
I seem to need to double tap it with two fingers. My Venu arrived from Amazon UK just two days ago. It seems to work when my index finger hits the screen right near where the 6 would be and my middle finger above it.
It's weird that it seems to take so much effort to get it to work.