Did you have much shrinkage, in length or width, of the canvas after applying the caulk?
When I painted my very large (6' x 24') artist's canvases (3 of them, two sea, one land) for a pirate game some years ago, I had a couple inches of shrink on the width and a LOT of shrinkage on the length. Piece was far too big to 'nail' down while painting.