This may not be an answer, but I had problems with that happening when I started and my instructor asked what my recipe was. In the process of telling her, she wondered if it was the type of powdered sugar I was using, generic brand made with beets. I switched and haven't had the problem since. I just didn't want to pay the extra during class because it was, after all, just for practice!
Some may disagree with me, and that's okay, but it may play a part of your problem. ??
It does sound as if there might have been some grease someplace. What icing bags were you using? For the royal, I use the disposable or parchment papers that way I know there is no grease residue anyplace.