Front Radius Problem

I just laid down my 1/8" birch inner skin last night and today. My the plywood flattened out between two spires on the front radius, making a near 1/2 gap in the middle. Argg!! I plan to skin with .040 aluminum and not convince the aluminum wont want to just take the same shape. I'm considering filling it up with plywood and bondo. Anyone encounter this problem?
Thanks, Jeff
Sorry, the pic orientation is rotated to the left 90 degrees...
Thanks, Jeff
Sorry, the pic orientation is rotated to the left 90 degrees...