Pencil lines can be very hard to get out depending on the pencil. Normal pencil graphite doesn't fade much over time either...
I learned to not mark with pencil myself -- I will draw on parchment paper and sew through the paper which can still transfer some of the graphite.
A fabric eraser, they are typically white and hard, can help some. You have to be careful to not scrub so hard you damage the quilting threads. Sometimes just a toothbrush can help get off some marks or improve the area.
There are other things you can do/try if you are willing to wash. Mixtures of a couple drops of Dawn Dishwasher detergent either just thinned with water or with some added ingredients like rubbing alcohol are possible. Even without washing, a light toothbrush scrub with water and a little rubbing alcohol might help (if it dries clear).