Assuming it's a kit, everything should fit snugly, eventually.

But, until you finally get those last few stitches in, and the bulkheads etc needed to give it its correct shape, it will bulge and sag and bits won't seem to fit.

Force is probably not needed. What helped me getting it to line up (on a Sassafras 16) was tokeep the stitches relatively loose until the last ones were in, then gradually tighten starting from the middle, and working towards the bow and stern evenly from there. 

I hope that helps.

I should clarify that I meant tightening in several sessions from middle to the ends. Loose at first, then middle to ends say "finger tight" (leaving some play in the wires), then repeat leaving only minor play, then finally tight enough to stop obvious movement. Never overtighten.

The idea is that the stitches all work together to form and hold the shape, and the tension should be as even as possible across the whole structure...

Thanks David, that makes sense. ive been starting at the bow and working aft but  the ends are fine its the middle thats the problem. ive been trying to work it out gradually, next session ill try it your way