A. It is possible that the heads of the screws are corroding, but this is unlikely if you are using corrosion-resistant screws. Stainless steel screws are best.
The most probable cause of the problem is condensation on the screw heads. The screws are good conductors of heat and are sunk well into the cold studs on an exterior wall. Since they are insulated only by wood, not by the cavity insulation, the screws can become one of the coldest objects in the room, and a thin film of condensation may form on...
to read the full article.