If the knob or the buttons ever stick, it's probably because they're binding up against the plastic fascia.

If you loosen the hex screws, you'll notice that the fascia has a little room to move around on its mountings. Sometimes this will press against the buttons or the knob. Spend a few minutes adjusting the tightness of the screws and the position of the fascia until everything works smoothly. Do not tighten the screws too hard, as doing so is a sure way to make the buttons stick. Just get them gently tight. Mine are only as tight as I can make them with my fingertip pressed against the end of the screw (I don't cinch them down with the hex wrench at all).

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Tony Fabris