I have an annoying problem with my 2012 Macbook Air, the trackpad "buttons" take about 4-5 (audible) clicks before the computer actually recognises that I have indeed made a click. Used to work fine, i.e. always on first click. Any suggestions? The machine is still under a 3 year warranty, but my nearest Apple Store is quite far away, so don't want to have to head over there, unless really necessary.