Hello Diana and welcome to the ArtRage forums
If your pencil shading layer is not set to Normal or is not at 100% Opacity it might be allowing something from a layer beneath to show through.
Similarly layers above it using different bland modes or Opacity's may be affecting what you see.
As those marks repeat in a very regular pattern, two things first spring to mind looking at your screenshot there;
1: Could they be related to the Canvas texture you have used? Canvas textures are often tiled ie repeated in order to cover the required area.
2: Do you have any layers using Blend modes other than Normal and/or reduced Opacity above your pencil shading? Something using a repeated fill pattern perhaps?
When I’m painting I always have layers set to Overlay at the top of the Layer stack to add more “texture” to the painting below. It might be worth going through your layers turning them On/Off one at a time to see if you can isolate a rouge layer that could be the cause.
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