Previously, my M331 left key was not sensitive just like what u describe. I tried dismantle it and applied some WD40 on the button sensor (red in color), using a stick. And it work again.
But this time, my scrolling wheel button was not working using this method. It remained unsensitive. I guess thats it, as I have used it for more than 5 to 6 yrs. Quite a good value for this mouse thaf it can sustain this long.