Problem with importing, missing faces

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.

3DS MaxThe next picture is after imported in UE4.

(3DS Max)

How am i supposed to do in this case? I´ve searched some others similar questions, bur anyone worked for me.

What happens if you set the material to 2 sided? Are those polys back-facing?

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)

In 3D Max?
They are normal-facing. Did nothing else. Modeled and texturized.

I changed Poly to Mesh and then apply Normal Modify, and worked. When you say Flip the normals, is that right?

Sorry for the late, but worked your solution.

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.