Some swarf jammed in the scroll plate? Had that happen on numerous occasions back in the machining days. Could just be something small stuck there or forced into the scroll thread, causes stiffness/complete stop in the jaws movement. Dismantle chuck, close inspection of inside may be all that's needed. DON'T lube with grease, very light oil if nothing at all, the stickier the lube, the more likely to gather more crap! Slippery, not sticky. (works fer change gears and the like too)