When a wet edge is not maintained while painting, lap marks form. It can also occur when the paint is only applied vertically. Maintain a wet edge and spread the paint on the wall in a "W" or "M" pattern. This will aid in the spread of the coating and the formation of an even film on the surface. After that, you can go back over that area and lightly roll from top to bottom to minimise "shadowing" in the dried film.