refiring piece of porcelain slip under glaze.

I have a cone 6 piece that when it was leather hard, I covered with porcelain slip to make the clay lighter in color. I fired it at 06 for bisque, and then glazed it, and fired to cone 6. It came out very rough, with the porcelain in little bubbles that looked smooth like it fired to temperature, with the glaze sort of under fired looking. The surface was really rough. I can't figure out what happened. My teacher does this all of the time with the same clay, but I took mine home and fired it. Has anyone had this problem? I have a fairly new Skutt with electronic controls, and the witness cone 6 slumped. Thanks, Martha