I had a problem with pad 13: The lower left corner would catch on edge of the faceplate cut out and stay down. Tapping it on the upper right corner or just wiggling it in the middle, would release it. It was this way out of the box, and made me .
I filed a support request, and they offered to immediately take it back for repair... but I wanted to hold on to it a few days first.
After about four days of playing on the unit, the key seems to have re-seated itself. It no longer sticks, even if I deliberately try to press it on the lower left corner. Yay !
So, if the sticking seems like the problem I had, you might just keep on tappin' for a few days and see...