This is really common.  I've had to apply brute force/tape/prying/pushing on each boat I've built (Shearwater and Wood Duck) as well as the strip-built I'm helping my neighbor with.  It seems on my builds it's been a combination of hull narrowing and deck flattening/widening. 

It really helps to have 2 people to do this step.  I start by taping the bow and stern using clear packing tape, then work my way toward the middle as far as I can (usually only a foot or so), then work in the cockpit area where there's more flexibility in the deck and you can easily reach inside to push the hull out while squeezing the deck.  Then there's the tricky sections between the cockpit and bow or stern where you can't easily reach.  I use a combination of wedging the hull open by pushing the appropriate hull forms forward or backward from the cockpit (unless you've already installed the bulkheads - then use a stick of appropriate length; glueing sandpaper to the ends helps keep it from slipping out as you push into place), and squeezing the deck together (helper does this) and use a putty knife to push into the deck/hull seam and pry in the appropriate direction (usually push handle up to pry hull out).  Then quickly apply packing tape. Work along applying tape every 6 inches or so until you have it. 

It is pretty hard to do sometimes, but if you're persistent it will come back together.  I actually crawled head-first into the Wood Duck 12 I'm currently building to push the hull form into place and nearly got stuck!  Fortunately I'm pretty flexible so I could get out again, but I had visions of my 15-year old son having to pull me out.  It must have looked pretty funny; good thing he didn't have a video camera rolling. 

Good luck,

Kathy

Paul

I'll second Kathy's recommendations.  I just finished a second woodduck hybrid and cosmetically it is very important for the deck to fit the hull with out much trimming.  I used four 1 by 2's and like Kathy hot glued small pieces of sand paper to get a good grip on the hull.  These could be called spreader bars.  in both boats the hull got narrower after taking the temporary bulkheads out of the hull.  I used the the spreader bars to get the hull back to the width that matched the deck and then used packing tape to secure the deck prior to using some thickened epoxy to tack the deck to the hull.  I credit Lou Farhood for this trick.  It seems a common issue at least for wood ducks but it is a solveable problem.

Good luck with the rest of the build.

Rod

Measure the width of the hull in comparison with the width of your forms. All those little strips of wood you bent around the forms are now trying to straighten back out pulling the sides of the hull inward. Use spreader sticks as above or modify/trim down a few stations to employ to reshape the hull during the joining process. Remember, after assembly, the spreader sticks or forms will have to be removed.

Lew

Well, it looks like I was way off.  After trying to put the forms back into the hull it appears that the hull is the culprit afterall, not the deck (which hasn't changed at all).  This afternoon will be spent making spacers.  With any luck, and a little bit of motivation, I'll be taping the seam by the end of the weekend and will have myself a functioning kayak (nevermind all that finishing stuff....).  Thanks for all of the advice!

Paul