After some several tests there is just one block the nanites can not be grinded. It's the light armor corner tip. The ninates just fly around this block, sometimes they grind the first 1 - 3 steelplates off but than they stopp. Between 1 - 2 streams try to grind it down, so if you grind a ship with some of these blocks it could be that there are just a few streams who work. The others fly around the light armor corner tips.
It's like they are waiting for a pre-worker who will start the grinding.