The washable markers are about the only thing I used to mark my quilts with. I have had a time or two that I found a marking or two that did not wash out completely the first time. As yngldy said, wash the quilt again. The slight markings that remained did come out the second time I washed
What I do now, is I pretreat.. (but the laundry pretreaters such as Shout, Spray-n-Wash are too expensive) I have a spray bottle that I fill half up with Dawn liquid dish soap then fill the rest up with water and shake well to mix. Then I just spray the markings with the watered down Dawn liquid..everything washes out now. I would suggest using a bigger bottle only because the smaller the bottle the smaller the spray ... and less that gets sprayed out. So it takes longer with a smaller bottle