Hi everyone!
When I uvmap, sometimes certain meshes are doing this strange thing of having one or more polygons plain grey. They also don't show on the UV map islands! I've encountered this problem twice before: once, I solved it splitting a polygon that had several vertices in more, simpler polygons; and the other time it was a rectangle and I just couldn't get what was going wrong.
I've no idea on why this is happening (in this specific case is also a triangle, so it's clearly not a matter of complexity) and I'm attaching a screenshot to show you the situation! Thank you for your support!