When you attached the sheeting, did you wet it first to help it bend, then glued it down while still wet? If so, then this might contribute to the problem.
The wood will expand when wet. If you glue it down in that condition, it will be locked in place, and when the wood dries, it will want to shrink back to original dimensions. It can't do that easily since it is glued down. So, it shrinks back as best it can and gives that hollow appearance.
If you wet it, hold it in place with tape or Ace bandage to shape it to contour and let it dry completely before gluing it down.
Maybe heavier wood, such as 7lb. or 8lb., will have less tendency to produce the problem you have.