I've got a "mature" Challenge EH3A (the tan colored one). My project is to standard 3HD a set that is about 1" thick, comprised of lightweight matte coated sheets (think textbooks printed in color) plus front and back 10 pt. C1S cover sheets. The problem is that the drill head stops descending at varying depths, leaving the bottom sheets not drilled through. I've tried drilling half a set at a time, slowing/quickening the rate of descent of the drill head, sharpened the bits and waxed them also, all with the same results. Any ideas?