So my ender 3 prints are coming out amazing after a few mods. However some top layers( especially flat ones) come out pretty thin and not even attached to the rest of the print. I didn't really ever have this problem before so maybe I accidentally clicked something in s3d but I'm not sure what it could be. The bottom layers come out perfect though.
Try using 'fill holes' option in Cura.
I kind of had the same issue, and I could see the infill lines. All the prints after that were fine though so I think it was just a weird glitch.
Try increasing your print temperature. It looks like you aren't pushing out quite enough plastic in the top layers, which are where the plastic cools the most rapidly.
First things first, check the layer settings in S3D (see attached .jpg) If you have the Top Solid Layers, Bottom Solid Layers, and Outlines or Shells set correctly (ie enough top layers to cover your infill) then proceed to deepsteve's comment and explore how to tweek your top layers even more.
Here's a link I found. Is this the issue you're having? https://forum.simplify3d.com/viewtopic.php?t=3331