I've got the same problem with the left button on my G400. Did some digging and it seems the button just simply is crap.
Think I'm going to switch to G502.
EDIT: Get some of that "dry" cleaner in a spray bottle and try that, fixed it for me the last time I had problems with the button. Just pry the button just enough to get the stuff inside, let it evaporate for 5 minutes or so.