hi, 

a picture of your 'problem' with your post can make it a lot easier to help....

that said, according to your post, this is a hybrid? (cedar strip deck, okoume plywood hull).

on the hybrids, your hatch is the hatch you cut out of the deck...so the 'curve' should match as it was cut from the deck.

without a picture its a bit tought to see what you are doing.  is there a chance you are trying to fit a hatch cover from a stitch and glue (non-hybrid deck) to a hybrid deck?  if you hatch cover is not the the same material as the deck...that may be a clue to the problem.

h

   On both of my strip builds, the hatch covers tended to flatten out after they were cut from the deck.  It was pretty easy to get them back to the proper bow by locking them tightly in place with the toggles.  I found that installing two toggles on each side really helped with this.  With the extra pressure on the side toggles, I also found that I had to add some extra weatherstrip under the center of the hatch.

I think it is due to 2 things. In my case I think working to slow between glassing the inside and glassing the out side. As the epoxy cures it shrinks and warps things. Also the wood can dry and warp.   I had the same problem on my wood ducks. I temporarly glued 2 strips of wood to the bottom of the hatcheswith a little more curve then the deck has. It does not look good but now that I have used the boats all summer and all the epoxy has cured I will now grind off these peaces and finish the bottom side.