I started recently learning how to use Blender to create 3D models, to use them later in Unity3D.
After finishing translating all the vertices, I noticed these "faces" were accidentally deformed or even deleted. So, I deleted them and remade them selecting the corresponding vertices and hitting the
Make Edge/Face button. The result is the screenshot posted here, and they look good from Blender.
However, when I export it to FBX file and insert it in Unity3D, this selected face and the one equivalent at the other side are completely invisible from any angle. I tried flipping normals but they still remain invisible.
I also tried to press CtrlShiftAltM just in case there was some undetected non-manifold geometry, as suggested in this answer from another question.
I barely can guess that this is probably because of the twisted geometry of the face itself, but I can't figure out in detail the true causes.
What kind of techniques may help to correct this issue?