Partially plating issues could be caused by a failure of the copper plating from adhering and then falling off or some other manufacturing error. Unplated planchets somehow missed the plating process, but this effect can also be achieved with certain science experiments; so a closer examination or taking it to an expert is in order.
A couple of things to remember is the coin without plating or missing partial plating was struck when the plating was missing and the surface should be relatively smooth. If it is porous or too grainy then the plating might have been artificially removed.
Also see Missing Clad Layer
Images courtesy of Heritage Auctions