When using Simplify3D, you may try referring to their awesome troubleshooting guide: https://www.simplify3d.com/support/print-quality-troubleshooting/#holes-and-gaps-in-the-top-layers 3 reasons (from the guide) for gaps in the top layers:
under-extrusion: you may try to increase extrusion multiplier to see if this helps
low infill percentage: not likely in your case if increasing number of top layers did not help
not enough top layers: you already tried increasing number of top layers.
In addition to that, you may change certain parameters (such as extrusion multiplier) for a given number of layers by using Simplify3D features. It can be helpful to avoid problems to for the rest of the model.