If your eggs aren't whipping into a marshmallow-like glossy foam, that means the egg whites have been mixed with some kind of fat. This could possibly be yolk, or residue in the bowl or utensils you used. If this happens, you will need to start again in order to make this version of buttercream. To be sure, rub a lemon wedge (or a paper towel dampened with white vinegar), all over the bowls, whisks, spatulas and other equipment you will be using to make the meringue base.