An externally applied coating has simply washed off. What would cause this to happen?

If you apply a water-based product in cold conditions (i.e. below 10ºC), loss of adhesion can occur and the coating may simply wash off when it rains. To rectify, thoroughly clean down the surfaces to remove all dirt, grease and surface contaminants. Scrape back all areas of poorly adhering or defective coatings to a firm edge and rub down to 'feather' broken edges. Dust off and re-apply coating.