parts of my mesh disappear ??? plz help

parts of my mesh disappear
the mesh looks ok in maya but when i import it to unreal some parts parts of my mesh disappear
like so

even in substance designer

please help.

Are the normals all facing out? On one sided meshes normals face in one direction you may need to flip those on the quarter panel that’s missing, it looks like it’s missing in that shot of substance designer too