A lot of slicers show this type of problem when walls are too thin for a minimum of 3 lines of filament. Without having your model or factory file, it's hard to know how thin that portion of your model is.
JoeJ's advice will sometimes work if the wall isn't too thin.
If the wall is too thin, then JoeJ's method doesn't work. In that case another way to solve this is to increase your Extrusion Width in Simplify3D so that each extruded line is a little thicker, just thick enough to fill the gap at the top.
For example, in many slicers, if a wall is 0.90 mm thick, but being printed with 2 extruded lines each at 0.40 mm thick, you'll sometimes see a 0.10 mm gap between the two lines of the 0.90 mm wall. In such a case, there's not enough space for infill. To solve this easily, you can increase your Extrusion Width from 0.40 mm to 0.45 mm. Two 0.45 mm extruded lines will make a 0.90 mm thick wall.
It's best to avoid using models that have walls that are so thin that this issue pops up. Such thin walls are going to break easily. 2 mm walls are the minimum I would recommend.
But if your wall isn't that thin, JoeJ's advice will work.