If you were hand quilting, how long were the marks in the fabric? I don't know for sure, but maybe the fact that I do machine quilting, and the marks are usually washed out in a few days would make a difference? And, like everything else in quilting, it pays to pre-test the marker on the fabric. Glad to know that Oxy-Clean will get out stubborn marks!
Also, since I usually have starched my fabrics, that probably helps the marks to come out, and that is something that hand quilters won't be doing!