After more investigation I have discovered that the lines that will not print are all horizontal polylines (in the direction that the printer carriage moves). The outline of the horizontal polyline is printed with very fine lines but there is no fill. The same happens with arrow heads on horizontal dimensions. Fine lines up to about 0.5mm real width are not a problem, but I am trying to print the walls of a house at 100mm thickness at a scale of 1:100. The problem only presents itself above real line thicknesses of about 0.5mm.