When calculating additive toolpaths, PowerMill considers only the surfaces that are visible to a plane projected down the Z-axis of a specified workplane. The visible surfaces are said to be in the Line of Sight of the workplane. If a surface is obscured and therefore not within the Line of Sight, it is not included in the calculation.
Surfaces with faces that are near-vertical to the Line of Sight 
can lead to sub-optimal toolpaths.
To prevent sub-optimal toolpaths try to create a toolpath from a different workplane, with a more suitable Line of Sight.