those are the worst and hardest to detect. In other words, the small areas that are adhered to the shell still give it the arch it needs but what surrounds sags. Those are absolutely the hardest to deal with and are a headache.
We just had a new year one hood and it looked great coming out of the box. It was exactly what you are describing but new, if you can believe that. It got mud almost covering the entire surface and primed twice....keep in mind this was a brand new hood.