I have the same sticky Tap button problem on my new HD Pro X. The button itself is made of, or covered with, a plastic/rubbery material very similar to the material covering the handle of a pair of pliers. If you slide your finger across the button, your finger will stick or drag on the material surface.
I assume this is the same problem you're having.
Tapping the button, unless I am completely dead center with a perfectly perpindicular light tap, the button will hesitate before it pops out. If the tap is from or on the side, the hesitation is pretty lengthy. A fast tap sequence can be virtually impossible.
When I called Sweetwater tech support, where I purchased it, the rep went to check if the one they had set up did the same thing. He called back and said it did. Between us we assumed it was a design flaw.
But if others don't have the problem, maybe it's time we both contact Line 6 support!!!
