There might be a chip stuck in the quill rack and pinion.
Dan, If you added a spacer or even a washer to the top of the spindle, make sure the OD is no larger than the female part of the splines on the spindle. During that last inch of down travel, the top of the spindle drops into the mating spline on the quill. If you use a washer or spacer between the top of spindle and the tightening nuts with to large a OD, it stops the spindle from dropping that last inch. Hope this helps, JR49