Several "recipes" for marking pencil removal through Google.
1/4 cup water, 3/4 cup rubbing alcohol and 7-8 drops Palmolive liquid detergent. OR: 3 oz rubbing alcohol, 2 tablespoons water, 3 drops ivory dish soap and apply with soft toothbrush - no need to rinse-dries quickly. Other suggestions included clear windex or 409 spray - gently rub and rinse. I have not tried any of these and none of them addresses a situation where you have already tried to get the marks out by machine washing it with little success. Do like the suggestions that you contact Fons and Porter. Surely they will have some suggestions. Let us know what works.