I´ve imported some meshes, but i don´t get why there are some faces missing while in UE4.
As you can see in the picture below, they are the same mesh, but mirrored.
Flip the normals in 3dsmax. (you can recognize them when you are selecting polygons and the selected ones are a bit darker/grayer reddish than the others)
Hey, i had the same problem with missing faces on mesh while in UE4, im using Maya.
To me the fix was to clean up mesh re-merge some verts , triangulate and export as an obj.
Now same mesh that displays correct in UE4 as an obj does not work as an fbx file.