Was the bowl dry before you put the chocolate in? I would expect it was, but even if there were a few drops of water I would not have thought that would be a problem. All I can say is that, when my wife was making a chocolate sauce to pour over profitteroles, the instructions said to melt the chocolate (which was plain) and that went OK, but the next step was to add a spoonful or so of water and stir. As soon as she did this, it all went crumbly. She panicked a bit, as there was no mention of this in the recipe, but I just carried on stirring and added more water, and it quite quickly went to a smooth sauce. Is your milk chocolate unusual in any way? Which country are you in? What does the ingredients list say about cocoa solids and cocoa butter?