at a guess, you should check the direction of the Normals in Blender before export.
a quick test in UE4 would be to look at your model from underneath to see if the missing triangles are visible from the other side
at a guess, you should check the direction of the Normals in Blender before export.
a quick test in UE4 would be to look at your model from underneath to see if the missing triangles are visible from the other side